BT Group plc (SEC Filing Page | XBRL Cloud Viewer | XBRL Instance)

Line	Label	Object Class	Period Type	Balance	Report ElementName
1	100000 - Document - Document and Entity Information	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DocumentDocumentAndEntityInformation
2	Implied Table	Table	*	*	implied:Table
3	Documentand Entity Information [Abstract]	Abstract			bt:DocumentandEntityInformationAbstract
4	Document Information [Line Items]	LineItems			dei:DocumentInformationLineItems
5	Document Type	Concept (Submission Type)	For Period		dei:DocumentType
6	Amendment Flag	Concept (Yes/No)	For Period		dei:AmendmentFlag
7	Amendment Description	Concept (Text/String)	For Period		dei:AmendmentDescription
8	Document Period End Date	Concept (Date)	For Period		dei:DocumentPeriodEndDate
9	Document Fiscal Year Focus	Concept (Year)	For Period		dei:DocumentFiscalYearFocus
10	Document Fiscal Period Focus	Concept (Fiscal Period)	For Period		dei:DocumentFiscalPeriodFocus
11	Trading Symbol	Concept (Text/String)	For Period		dei:TradingSymbol
12	Entity Registrant Name	Concept (Text/String)	For Period		dei:EntityRegistrantName
13	Entity Central Index Key	Concept (CIK Number)	For Period		dei:EntityCentralIndexKey
14	Current Fiscal Year End Date	Concept (Month/Day)	For Period		dei:CurrentFiscalYearEndDate
15	Entity Well-known Seasoned Issuer	Concept (Yes/No)	For Period		dei:EntityWellKnownSeasonedIssuer
16	Entity Current Reporting Status	Concept (Yes/No)	For Period		dei:EntityCurrentReportingStatus
17	Entity Filer Category	Concept (Filer Category)	For Period		dei:EntityFilerCategory
18	Entity Common Stock, Shares Outstanding	Concept (Shares)	As Of		dei:EntityCommonStockSharesOutstanding
19	100010 - Statement - Group income statement	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/StatementGroupIncomeStatement
20	Statements [Table]	Table	*	*	bt:StatementsTable
21	Retrospective application and retrospective restatement [axis]	Axis			ifrs-full:RetrospectiveApplicationAndRetrospectiveRestatementAxis
22	Currently stated [member]	Member			ifrs-full:RestatedMember
23	Before Specific Items [Member]	Member			bt:BeforeSpecificItemsMember
24	Specific Items [Member]	Member			bt:SpecificItemsMember
25	Statements [Line Items]	LineItems			bt:StatementsLineItems
26	Revenue	Concept (Monetary)	For Period	Credit	ifrs-full:Revenue
27	Operating costs	Concept (Monetary)	For Period	Debit	ifrs-full:OperatingExpense
28	Operating profit (loss)	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLossFromOperatingActivities
29	Finance expense	Concept (Monetary)	For Period	Debit	ifrs-full:FinanceCosts
30	Finance income	Concept (Monetary)	For Period	Credit	ifrs-full:FinanceIncome
31	Net finance expense	Concept (Monetary)	For Period	Credit	ifrs-full:FinanceIncomeCost
32	Share of post tax profit (loss) of associates and joint ventures	Concept (Monetary)	For Period	Credit	ifrs-full:ShareOfProfitLossOfAssociatesAndJointVenturesAccountedForUsingEquityMethod
33	Profit (loss) before taxation	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLossBeforeTax
34	Taxation	Concept (Monetary)	For Period	Debit	ifrs-full:IncomeTaxExpenseContinuingOperations
35	Profit (loss) for the year	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLoss
36	Earnings per share	Abstract			ifrs-full:EarningsPerShareAbstract
37	Basic	Concept (Share)	For Period		ifrs-full:BasicEarningsLossPerShare
38	Diluted	Concept (Share)	For Period		ifrs-full:DilutedEarningsLossPerShare
39	100020 - Statement - Group statement of comprehensive income	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/StatementGroupStatementOfComprehensiveIncome
40	Implied Table	Table	*	*	implied:Table
41	Statement Of Comprehensive Income [Abstract]	Abstract			ifrs-full:StatementOfComprehensiveIncomeAbstract
42	Profit for the year	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLoss
43	Other comprehensive income (loss)	Abstract			ifrs-full:OtherComprehensiveIncomeAbstract
44	Items that will not be reclassified to the income statement	Abstract			ifrs-full:ComponentsOfOtherComprehensiveIncomeThatWillNotBeReclassifiedToProfitOrLossBeforeTaxAbstract
45	Remeasurements of the net pension obligation	Concept (Monetary)	For Period	Credit	ifrs-full:OtherComprehensiveIncomeBeforeTaxGainsLossesOnRemeasurementsOfDefinedBenefitPlans
46	Tax on pension remeasurements	Concept (Monetary)	For Period	Debit	ifrs-full:IncomeTaxRelatingToRemeasurementsOfDefinedBenefitPlansOfOtherComprehensiveIncome
47	Items that have been or may be reclassified to the income statement	Abstract			ifrs-full:ComponentsOfOtherComprehensiveIncomeThatWillBeReclassifiedToProfitOrLossBeforeTaxAbstract
48	Exchange differences on translation of foreign operations	Concept (Monetary)	For Period	Credit	ifrs-full:OtherComprehensiveIncomeBeforeTaxExchangeDifferencesOnTranslation
49	Fair value movements on available-for-sale assets	Concept (Monetary)	For Period	Credit	ifrs-full:OtherComprehensiveIncomeBeforeTaxAvailableforsaleFinancialAssets
50	Fair value movements on cash flow hedges:	Abstract			ifrs-full:CashFlowHedgesAbstract
51	� net fair value gains (losses)	Concept (Monetary)	For Period	Credit	ifrs-full:OtherComprehensiveIncomeBeforeTaxCashFlowHedges
52	� recognised in income and expense	Concept (Monetary)	For Period	Credit	ifrs-full:GainsLossesOnCashFlowHedgesBeforeTax
53	Tax on components of other comprehensive income that have been or may be reclassified	Concept (Monetary)	For Period	Debit	ifrs-full:IncomeTaxRelatingToComponentsOfOtherComprehensiveIncomeThatWillBeReclassifiedToProfitOrLoss
54	Other comprehensive income (loss) for the year, net of tax	Concept (Monetary)	For Period	Credit	ifrs-full:OtherComprehensiveIncome
55	Total comprehensive income (loss) for the year	Concept (Monetary)	For Period	Credit	ifrs-full:ComprehensiveIncome
56	100030 - Statement - Group balance sheet	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/StatementGroupBalanceSheet
57	Implied Table	Table	*	*	implied:Table
58	Statement Of Financial Position [Abstract]	Abstract			ifrs-full:StatementOfFinancialPositionAbstract
59	Non-current assets	Abstract			ifrs-full:NoncurrentAssetsAbstract
60	Intangible assets	Concept (Monetary)	As Of	Debit	ifrs-full:IntangibleAssetsAndGoodwill
61	Property, plant and equipment	Concept (Monetary)	As Of	Debit	ifrs-full:PropertyPlantAndEquipment
62	Derivative financial instruments	Concept (Monetary)	As Of	Debit	ifrs-full:NoncurrentDerivativeFinancialAssets
63	Investments	Concept (Monetary)	As Of	Debit	ifrs-full:NoncurrentInvestmentsOtherThanInvestmentsAccountedForUsingEquityMethod
64	Associates and joint ventures	Concept (Monetary)	As Of	Debit	ifrs-full:InvestmentsInSubsidiariesJointVenturesAndAssociates
65	Trade and other receivables	Concept (Monetary)	As Of	Debit	ifrs-full:NoncurrentReceivables
66	Deferred tax assets	Concept (Monetary)	As Of	Debit	ifrs-full:DeferredTaxAssets
67	Non current assets	Concept (Monetary)	As Of	Debit	ifrs-full:NoncurrentAssets
68	Current assets	Abstract			ifrs-full:CurrentAssetsAbstract
69	Programme rights	Concept (Monetary)	As Of	Debit	ifrs-full:CopyrightsPatentsAndOtherIndustrialPropertyRightsServiceAndOperatingRights
70	Inventories	Concept (Monetary)	As Of	Debit	ifrs-full:Inventories
71	Trade and other receivables	Concept (Monetary)	As Of	Debit	ifrs-full:TradeAndOtherCurrentReceivables
72	Current tax receivable	Concept (Monetary)	As Of	Debit	ifrs-full:CurrentTaxAssetsCurrent
73	Derivative financial instruments	Concept (Monetary)	As Of	Debit	ifrs-full:CurrentDerivativeFinancialAssets
74	Investments	Concept (Monetary)	As Of	Debit	ifrs-full:CurrentInvestments
75	Cash and cash equivalents	Concept (Monetary)	As Of	Debit	ifrs-full:CashAndCashEquivalents
76	Current assets	Concept (Monetary)	As Of	Debit	ifrs-full:CurrentAssets
77	Current liabilities	Abstract			ifrs-full:CurrentLiabilitiesAbstract
78	Loans and other borrowings	Concept (Monetary)	As Of	Credit	ifrs-full:ShorttermBorrowings
79	Derivative financial instruments	Concept (Monetary)	As Of	Credit	ifrs-full:CurrentDerivativeFinancialLiabilities
80	Trade and other payables	Concept (Monetary)	As Of	Credit	ifrs-full:TradeAndOtherCurrentPayables
81	Current tax liabilities	Concept (Monetary)	As Of	Credit	ifrs-full:CurrentTaxLiabilitiesCurrent
82	Provisions	Concept (Monetary)	As Of	Credit	ifrs-full:CurrentProvisions
83	Current liabilities	Concept (Monetary)	As Of	Credit	ifrs-full:CurrentLiabilities
84	Total assets less current liabilities	Concept (Monetary)	As Of	Debit	ifrs-full:AssetsLessCurrentLiabilities
85	Non-current liabilities	Abstract			ifrs-full:NoncurrentLiabilitiesAbstract
86	Loans and other borrowings	Concept (Monetary)	As Of	Credit	ifrs-full:LongtermBorrowings
87	Derivative financial instruments	Concept (Monetary)	As Of	Credit	ifrs-full:NoncurrentDerivativeFinancialLiabilities
88	Retirement benefit obligations	Concept (Monetary)	As Of	Credit	ifrs-full:NoncurrentRecognisedLiabilitiesDefinedBenefitPlan
89	Other payables	Concept (Monetary)	As Of	Credit	ifrs-full:NoncurrentPayables
90	Deferred tax liabilities	Concept (Monetary)	As Of	Credit	ifrs-full:DeferredTaxLiabilities
91	Provisions	Concept (Monetary)	As Of	Credit	ifrs-full:NoncurrentProvisions
92	Non current liabilities	Concept (Monetary)	As Of	Credit	ifrs-full:NoncurrentLiabilities
93	Equity	Abstract			ifrs-full:EquityAbstract
94	Ordinary shares	Concept (Monetary)	As Of	Credit	ifrs-full:IssuedCapital
95	Share premium	Concept (Monetary)	As Of	Credit	ifrs-full:SharePremium
96	Own shares	Concept (Monetary)	As Of	Debit	ifrs-full:TreasuryShares
97	Merger reserve	Concept (Monetary)	As Of	Credit	ifrs-full:MergerReserve
98	Other reserves	Concept (Monetary)	As Of	Credit	ifrs-full:OtherReserves
99	Retained profit (loss)	Concept (Monetary)	As Of	Credit	ifrs-full:RetainedEarnings
100	Total equity	Concept (Monetary)	As Of	Credit	ifrs-full:Equity
101	Total equity and Liabilities	Concept (Monetary)	As Of	Credit	ifrs-full:EquityAndLiabilities
102	100040 - Statement - Group statement of changes in equity	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/StatementGroupStatementOfChangesInEquity
103	Statements [Table]	Table	*	*	bt:StatementsTable
104	Retained (Loss) Earnings [Member]	Axis			ifrs-full:ClassesOfShareCapitalAxis
105	Share capital [member]	Member			ifrs-full:ClassesOfShareCapitalMember
106	Share Capital [Member]	Member			ifrs-full:IssuedCapitalMember
107	Share Premium [Member]	Member			ifrs-full:SharePremiumMember
108	Own Shares [Member]	Member			ifrs-full:TreasurySharesMember
109	Merger Reserve [Member]	Member			ifrs-full:MergerReserveMember
110	Other Reserves [Member]	Member			ifrs-full:OtherReservesMember
111	Retained (Loss) Earnings [Member]	Member			ifrs-full:RetainedEarningsMember
112	Statements [Line Items]	LineItems			bt:StatementsLineItems
113	Beginning balance	Concept (Monetary)	As Of	Credit	ifrs-full:Equity
114	Profit for the year	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLoss
115	Other comprehensive income (loss) � before tax	Concept (Monetary)	For Period	Credit	ifrs-full:OtherComprehensiveIncomeBeforeTax
116	Tax on other comprehensive income (loss)	Concept (Monetary)	For Period		ifrs-full:IncomeTaxRelatingToComponentsOfOtherComprehensiveIncome
117	Transferred to the income statement	Concept (Monetary)	For Period	Credit	ifrs-full:GainsLossesOnCashFlowHedgesBeforeTax
118	Total comprehensive income (loss) for the year	Concept (Monetary)	For Period	Credit	ifrs-full:ComprehensiveIncome
119	Issue of new shares	Concept (Monetary)	For Period	Credit	bt:StockIssuedDuringPeriodValueNewIssues1
120	Transfers to realised profit	Concept (Monetary)	For Period	Debit	bt:TransfersToRealisedProfit
121	Dividends to shareholders	Concept (Monetary)	For Period	Debit	ifrs-full:DividendsPaid
122	Share-based payments	Concept (Monetary)	For Period	Credit	ifrs-full:IncreaseDecreaseThroughSharebasedPaymentTransactions
123	Tax on share-based payments	Concept (Monetary)	For Period	Debit	ifrs-full:DecreaseIncreaseThroughTaxOnSharebasedPaymentTransactions
124	Net buyback of own shares	Concept (Monetary)	For Period	Debit	ifrs-full:PurchaseOfTreasuryShares
125	Other movements	Concept (Monetary)	For Period	Credit	bt:OtherMovements
126	Ending balance	Concept (Monetary)	As Of	Credit	ifrs-full:Equity
127	100050 - Statement - Group statement of changes in equity (Parenthetical)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/StatementGroupStatementOfChangesInEquityParenthetical
128	Statements [Table]	Table	*	*	bt:StatementsTable
129	Retained (Loss) Earnings [Member]	Axis			ifrs-full:ClassesOfShareCapitalAxis
130	Share capital [member]	Member			ifrs-full:ClassesOfShareCapitalMember
131	Merger Reserve [Member]	Member			ifrs-full:MergerReserveMember
132	Statements [Line Items]	LineItems			bt:StatementsLineItems
133	Ordinary shares	Concept (Monetary)	As Of	Credit	ifrs-full:IssuedCapital
134	Number of ordinary shares	Concept (Shares)	As Of		bt:NumberOfSharesOutstandingIncludingTreasuryStock
135	Par value per share	Concept (Share)	As Of		ifrs-full:ParValuePerShare
136	Number of shares issued	Concept (Shares)	For Period		bt:StockIssuedDuringPeriodSharesNewIssues1
137	Issue of shares at market price	Concept (Share)	As Of		bt:IssueOfSharesAtMarketPricePerShare
138	Increase in merger reserve from share issuance	Concept (Monetary)	For Period	Credit	bt:IncreaseInMergerReserveFromShareIssuance
139	Issue costs	Concept (Monetary)	For Period	Debit	ifrs-full:ShareIssueRelatedCost
140	Transfer of merger reserve	Concept (Monetary)	For Period	Debit	bt:TransferOfMergerReserveToRealisedProfit
141	Proceeds from share issuance	Concept (Monetary)	For Period	Debit	ifrs-full:ProceedsFromIssueOfOrdinaryShares
142	Increase in share capital	Concept (Monetary)	For Period	Credit	ifrs-full:ChangesInEquity
143	100060 - Statement - Group cash flow statement	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/StatementGroupCashFlowStatement
144	Implied Table	Table	*	*	implied:Table
145	Statement Of Cash Flows [Abstract]	Abstract			ifrs-full:StatementOfCashFlowsAbstract
146	Cash flow from operating activities	Abstract			ifrs-full:CashFlowsFromUsedInOperatingActivitiesAbstract
147	Profit (loss) before taxation	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLossBeforeTax
148	Share of post tax loss (profit) of associates and joint ventures	Concept (Monetary)	For Period	Credit	ifrs-full:ShareOfProfitLossOfAssociatesAndJointVenturesAccountedForUsingEquityMethod
149	Net finance expense	Concept (Monetary)	For Period	Credit	ifrs-full:FinanceIncomeCost
150	Operating profit (loss)	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLossFromOperatingActivities
151	Other non-cash charges	Concept (Monetary)	For Period	Debit	ifrs-full:OtherAdjustmentsForNoncashItems
152	Profit on disposal of businesses	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitsLossesOnDisposalOfInvestmentsAndChangesInValueOfInvestments
153	Depreciation and amortisation	Concept (Monetary)	For Period	Debit	ifrs-full:AdjustmentsForDepreciationAndAmortisationExpense
154	Increase in inventories	Concept (Monetary)	For Period	Debit	ifrs-full:AdjustmentsForDecreaseIncreaseInInventories
155	Increase in programme rights	Concept (Monetary)	For Period	Debit	bt:AdjustmentsForDecreaseIncreaseInProgrammeRights
156	(Increase) decrease in trade and other receivables	Concept (Monetary)	For Period	Debit	ifrs-full:AdjustmentsForDecreaseIncreaseInTradeAccountReceivable
157	(Decrease) increase in trade and other payables	Concept (Monetary)	For Period	Debit	ifrs-full:AdjustmentsForIncreaseDecreaseInTradeAccountPayable
158	Decrease in other liabilities	Concept (Monetary)	For Period	Debit	ifrs-full:AdjustmentsForIncreaseDecreaseInOtherLiabilities
159	(Decrease) increase in provisions	Concept (Monetary)	For Period	Debit	ifrs-full:AdjustmentsForProvisions
160	Cash generated from operations	Concept (Monetary)	For Period		ifrs-full:CashFlowsFromUsedInOperations
161	Income taxes paid	Concept (Monetary)	For Period	Debit	ifrs-full:AdjustmentsForIncomeTaxExpense
162	Net cash inflow from operating activities	Concept (Monetary)	For Period		ifrs-full:CashFlowsFromUsedInOperatingActivities
163	Cash flow from investing activities	Abstract			ifrs-full:CashFlowsFromUsedInInvestingActivitiesAbstract
164	Interest received	Concept (Monetary)	For Period	Debit	ifrs-full:InterestReceivedClassifiedAsInvestingActivities
165	Dividends received from associates and joint ventures	Concept (Monetary)	For Period	Debit	ifrs-full:DividendsReceivedFromInvestmentsAccountedForUsingEquityMethodClassifiedAsInvestingActivities
166	Acquisition of subsidiaries	Concept (Monetary)	For Period	Credit	ifrs-full:CashFlowsUsedInObtainingControlOfSubsidiariesOrOtherBusinessesClassifiedAsInvestingActivities
167	Proceeds on disposal of subsidiariesd, associates and joint ventures	Concept (Monetary)	For Period	Debit	ifrs-full:ProceedsFromSalesOfInvestmentsAccountedForUsingEquityMethod
168	Acquisition of joint ventures	Concept (Monetary)	For Period	Credit	ifrs-full:OtherCashPaymentsToAcquireInterestsInJointVenturesClassifiedAsInvestingActivities
169	Proceeds on disposal of current financial assets	Concept (Monetary)	For Period	Debit	ifrs-full:ProceedsFromSalesOrMaturityOfFinancialInstrumentsClassifiedAsInvestingActivities
170	Purchases of current financial assets	Concept (Monetary)	For Period	Credit	ifrs-full:PurchaseOfFinancialInstrumentsClassifiedAsInvestingActivities
171	Proceeds on disposal of non-current asset investments	Concept (Monetary)	For Period	Debit	ifrs-full:ProceedsFromDisposalOfNoncurrentAssetsOrDisposalGroupsClassifiedAsHeldForSaleAndDiscontinuedOperations
172	Purchases of non-current asset investments	Concept (Monetary)	For Period	Debit	ifrs-full:OtherCashReceiptsFromSalesOfEquityOrDebtInstrumentsOfOtherEntitiesClassifiedAsInvestingActivities
173	Proceeds on disposal of property, plant and equipment	Concept (Monetary)	For Period	Debit	ifrs-full:ProceedsFromSalesOfPropertyPlantAndEquipmentClassifiedAsInvestingActivities
174	Purchases of property, plant and equipment and software	Concept (Monetary)	For Period	Credit	ifrs-full:PurchaseOfPropertyPlantAndEquipmentClassifiedAsInvestingActivities
175	Net cash outflow from investing activities	Concept (Monetary)	For Period	Debit	ifrs-full:CashFlowsFromUsedInInvestingActivities
176	Cash flow from financing activities	Abstract			ifrs-full:CashFlowsFromUsedInFinancingActivitiesAbstract
177	Equity dividends paid	Concept (Monetary)	For Period	Credit	ifrs-full:DividendsPaidClassifiedAsFinancingActivities
178	Interest paid	Concept (Monetary)	For Period	Credit	ifrs-full:InterestPaidClassifiedAsFinancingActivities
179	Repayment of borrowings	Concept (Monetary)	For Period	Credit	ifrs-full:RepaymentsOfBorrowingsClassifiedAsFinancingActivities
180	Proceeds from bank loans and bonds	Concept (Monetary)	For Period	Debit	ifrs-full:ProceedsFromBorrowingsClassifiedAsFinancingActivities
181	Cash flows from derivatives related to net debt	Concept (Monetary)	For Period	Credit	bt:CashFlowsFromDerivativesRelatedToNetDebtClassifiedAsFinancingActivities
182	Drawdown on acquisition facility	Concept (Monetary)	For Period	Debit	bt:DrawdownOnAcquisitionFacility
183	Repayment of acquisition facility	Concept (Monetary)	For Period	Credit	bt:RepaymentOfAcquisitionFacility
184	Repayment of EE revolving credit facility	Concept (Monetary)	For Period	Credit	bt:RepaymentsOfRevolvingCreditFacility
185	Proceeds from issue of own shares	Concept (Monetary)	For Period	Debit	ifrs-full:ProceedsFromSaleOrIssueOfTreasuryShares
186	Repurchase of ordinary share capital	Concept (Monetary)	For Period	Credit	bt:RepurchaseOfOrdinaryShareCapital
187	Net cash (outflow) inflow from financing activities	Concept (Monetary)	For Period	Debit	ifrs-full:CashFlowsFromUsedInFinancingActivities
188	Net increase in cash and cash equivalents	Concept (Monetary)	For Period	Debit	ifrs-full:IncreaseDecreaseInCashAndCashEquivalentsBeforeEffectOfExchangeRateChanges
189	Opening cash and cash equivalents	Concept (Monetary)	As Of	Debit	ifrs-full:CashAndCashEquivalentsIfDifferentFromStatementOfFinancialPosition
190	Net increase in cash and cash equivalents	Concept (Monetary)	For Period	Debit	ifrs-full:IncreaseDecreaseInCashAndCashEquivalentsBeforeEffectOfExchangeRateChanges
191	Effect of exchange rate changes	Concept (Monetary)	For Period	Debit	ifrs-full:EffectOfExchangeRateChangesOnCashAndCashEquivalents
192	Closing cash and cash equivalents	Concept (Monetary)	As Of	Debit	ifrs-full:CashAndCashEquivalentsIfDifferentFromStatementOfFinancialPosition
193	100070 - Statement - Group cash flow statement (Parenthetical)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/StatementGroupCashFlowStatementParenthetical
194	Implied Table	Table	*	*	implied:Table
195	Statement Of Cash Flows [Abstract]	Abstract			ifrs-full:StatementOfCashFlowsAbstract
196	Pension deficit payments	Concept (Monetary)	For Period	Credit	bt:PensionDeficitPaymentsIncludedInOtherLiabilities
197	Prepayments in respect of acquisition of intangible assets	Concept (Monetary)	As Of	Debit	bt:PrepaymentsInRespectOfAcquisitionOfIntangibleAssets
198	Purchase price allocation true-up of consideration in acquisition	Concept (Monetary)	For Period	Credit	bt:PurchasePriceAllocationTrueupOfConsiderationInAcquisition
199	Net of bank overdrafts	Concept (Monetary)	As Of	Credit	ifrs-full:BankOverdraftsClassifiedAsCashEquivalents
200	100080 - Statement - Group plc balance sheet	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/StatementGroupPlcBalanceSheet
201	Statements [Table]	Table	*	*	bt:StatementsTable
202	Categories of related parties [axis]	Axis			ifrs-full:CategoriesOfRelatedPartiesAxis
203	Entity's total for related parties [member]	Member			ifrs-full:EntitysTotalForRelatedPartiesMember
204	BT Group plc [Member]	Member			ifrs-full:ParentMember
205	Statements [Line Items]	LineItems			bt:StatementsLineItems
206	Non-current assets	Abstract			ifrs-full:NoncurrentAssetsAbstract
207	Investments	Concept (Monetary)	As Of	Debit	ifrs-full:NoncurrentInvestmentsOtherThanInvestmentsAccountedForUsingEquityMethod
208	Trade and other receivables	Concept (Monetary)	As Of	Debit	ifrs-full:NoncurrentReceivables
209	Non current assets	Concept (Monetary)	As Of	Debit	ifrs-full:NoncurrentAssets
210	Current assets	Abstract			ifrs-full:CurrentAssetsAbstract
211	Trade and other receivables	Concept (Monetary)	As Of	Debit	ifrs-full:TradeAndOtherCurrentReceivables
212	Cash and cash equivalents	Concept (Monetary)	As Of	Debit	ifrs-full:CashAndCashEquivalents
213	Current assets	Concept (Monetary)	As Of	Debit	ifrs-full:CurrentAssets
214	Current liabilities	Abstract			ifrs-full:CurrentLiabilitiesAbstract
215	Trade and other payables	Concept (Monetary)	As Of	Credit	ifrs-full:TradeAndOtherCurrentPayables
216	Current liabilities	Concept (Monetary)	As Of	Credit	ifrs-full:CurrentLiabilities
217	Total assets less current liabilities	Concept (Monetary)	As Of	Debit	ifrs-full:AssetsLessCurrentLiabilities
218	Non-current liabilities	Abstract			ifrs-full:NoncurrentLiabilitiesAbstract
219	Loans and other borrowings	Concept (Monetary)	As Of	Credit	ifrs-full:LongtermBorrowings
220	Non current liabilities	Concept (Monetary)	As Of	Credit	ifrs-full:NoncurrentLiabilities
221	Equity	Abstract			ifrs-full:EquityAbstract
222	Ordinary shares	Concept (Monetary)	As Of	Credit	ifrs-full:IssuedCapital
223	Share premium	Concept (Monetary)	As Of	Credit	ifrs-full:SharePremium
224	Capital redemption reserve	Concept (Monetary)	As Of	Credit	ifrs-full:CapitalRedemptionReserve
225	Merger reserve	Concept (Monetary)	As Of	Credit	ifrs-full:MergerReserve
226	Own shares	Concept (Monetary)	As Of	Debit	ifrs-full:TreasuryShares
227	Retained profit	Concept (Monetary)	As Of	Credit	ifrs-full:RetainedEarnings
228	Total equity	Concept (Monetary)	As Of	Credit	ifrs-full:Equity
229	Total equity and Liabilities	Concept (Monetary)	As Of	Credit	ifrs-full:EquityAndLiabilities
230	100090 - Statement - Group plc balance sheet (Parenthetical)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/StatementGroupPlcBalanceSheetParenthetical
231	Statements [Table]	Table	*	*	bt:StatementsTable
232	Categories of related parties [axis]	Axis			ifrs-full:CategoriesOfRelatedPartiesAxis
233	Entity's total for related parties [member]	Member			ifrs-full:EntitysTotalForRelatedPartiesMember
234	BT Group plc [Member]	Member			ifrs-full:ParentMember
235	Types of interest rates [axis]	Axis			ifrs-full:TypesOfInterestRatesAxis
236	Interest rate types [member]	Member			ifrs-full:InterestRateTypesMember
237	L I B O R	Member			bt:LIBORMember
238	Maturity [axis]	Axis			ifrs-full:MaturityAxis
239	Aggregated time bands [member]	Member			ifrs-full:AggregatedTimeBandsMember
240	Loans Repayable on 31 January 2058 [Member]	Member			bt:LoansDueJanuaryThirtyOneTwoThousandFiftyEightMember
241	Loans Repayable on 21 December 2064 [Member]	Member			bt:LoansDueDecemberTwentyOneTwoThousandSixtyFourMember
242	Statements [Line Items]	LineItems			bt:StatementsLineItems
243	Receivables related to equity placing	Concept (Monetary)	As Of	Debit	bt:ReceivablesFromEquityPlacing
244	Proceeds from sale of investment	Concept (Monetary)	For Period	Debit	bt:ProceedsFromSaleOfInvestment
245	Share issue related cost	Concept (Monetary)	For Period	Debit	ifrs-full:ShareIssueRelatedCost
246	Loans received	Concept (Monetary)	As Of	Credit	ifrs-full:LoansReceived
247	Loans receivable	Concept (Monetary)	As Of	Debit	ifrs-full:CorporateLoans
248	Borrowings, adjustment to interest rate basis	Concept (Percent)	As Of		ifrs-full:BorrowingsAdjustmentToInterestRateBasis
249	Accrued interest included in current trade and other receivables	Concept (Monetary)	As Of	Debit	bt:AccruedInterest
250	Trade and other payable relating to loan from group undertakings	Concept (Monetary)	As Of	Credit	ifrs-full:TradeAndOtherPayablesToRelatedParties
251	Trade and other payable relating to other creditors	Concept (Monetary)	As Of	Credit	bt:TradeAndOtherPayablesToOtherCreditors
252	Loans and other borrowings from group undertakings	Concept (Monetary)	As Of	Credit	ifrs-full:LongtermBorrowings
253	Profit for the year	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLoss
254	100100 - Statement - Group plc statement of changes in equity	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/StatementGroupPlcStatementOfChangesInEquity
255	Statements [Table]	Table	*	*	bt:StatementsTable
256	Categories of related parties [axis]	Axis			ifrs-full:CategoriesOfRelatedPartiesAxis
257	Entity's total for related parties [member]	Member			ifrs-full:EntitysTotalForRelatedPartiesMember
258	BT Group plc [Member]	Member			ifrs-full:ParentMember
259	Retained (Loss) Earnings [Member]	Axis			ifrs-full:ClassesOfShareCapitalAxis
260	Share capital [member]	Member			ifrs-full:ClassesOfShareCapitalMember
261	Share Capital [Member]	Member			ifrs-full:IssuedCapitalMember
262	Share Premium [Member]	Member			ifrs-full:SharePremiumMember
263	Capital Redemption Reserve [Member]	Member			ifrs-full:CapitalRedemptionReserveMember
264	Merger Reserve [Member]	Member			ifrs-full:MergerReserveMember
265	Own Shares [Member]	Member			ifrs-full:TreasurySharesMember
266	Retained (Loss) Earnings [Member]	Member			ifrs-full:RetainedEarningsMember
267	Statements [Line Items]	LineItems			bt:StatementsLineItems
268	Beginning balance	Concept (Monetary)	As Of	Credit	ifrs-full:Equity
269	Profit for the year	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLoss
270	Transfer to realised profit	Concept (Monetary)	For Period	Debit	bt:TransfersToRealisedProfit
271	Dividends to shareholders	Concept (Monetary)	For Period	Debit	ifrs-full:DividendsPaid
272	Share-based payments	Concept (Monetary)	For Period	Credit	ifrs-full:IncreaseDecreaseThroughSharebasedPaymentTransactions
273	Net buyback of own shares	Concept (Monetary)	For Period	Debit	ifrs-full:PurchaseOfTreasuryShares
274	Ending balance	Concept (Monetary)	As Of	Credit	ifrs-full:Equity
275	100110 - Statement - Group plc statement of changes in equity (Parenthetical)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/StatementGroupPlcStatementOfChangesInEquityParenthetical
276	Statements [Table]	Table	*	*	bt:StatementsTable
277	Categories of related parties [axis]	Axis			ifrs-full:CategoriesOfRelatedPartiesAxis
278	Entity's total for related parties [member]	Member			ifrs-full:EntitysTotalForRelatedPartiesMember
279	BT Group plc [Member]	Member			ifrs-full:ParentMember
280	Retained (Loss) Earnings [Member]	Axis			ifrs-full:ClassesOfShareCapitalAxis
281	Share capital [member]	Member			ifrs-full:ClassesOfShareCapitalMember
282	Own Shares [Member]	Member			bt:OwnPremiumMember
283	Statements [Line Items]	LineItems			bt:StatementsLineItems
284	Ordinary shares	Concept (Monetary)	As Of	Credit	ifrs-full:IssuedCapital
285	Number of ordinary shares	Concept (Shares)	As Of		bt:NumberOfSharesOutstandingIncludingTreasuryStock
286	Par value per share	Concept (Share)	As Of		ifrs-full:ParValuePerShare
287	Number of shares issued	Concept (Shares)	As Of		ifrs-full:NumberOfSharesIssued
288	Issue costs	Concept (Monetary)	For Period	Debit	bt:ShareIssueCostRelatedToOwnShareIssue
289	Shares held at cost	Concept (Shares)	As Of		bt:NumberOfSharesHeldAtCost
290	Aggregate nominal value of shares held at cost	Concept (Monetary)	As Of	Credit	bt:AggregateNominalValueOfSharesHeldAtCost
291	Profit for the year	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLoss
292	100120 - Disclosure - Basis of presentation	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureBasisOfPresentation
293	Implied Table	Table	*	*	implied:Table
294	Basis Of Preparation [Abstract]	Abstract			bt:BasisOfPreparationAbstract
295	Basis of preparation	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfBasisOfPreparationOfFinancialStatementsExplanatory
296	100130 - Disclosure - Critical accounting estimates and key judgements	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureCriticalAccountingEstimatesAndKeyJudgements
297	Implied Table	Table	*	*	implied:Table
298	Critical Accounting Estimates And Key Judgements [Abstract]	Abstract			bt:CriticalAccountingEstimatesAndKeyJudgementsAbstract
299	Critical accounting estimates and key judgements	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfAccountingJudgementsAndEstimatesExplanatory
300	100140 - Disclosure - Significant accounting policies	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureSignificantAccountingPolicies
301	Implied Table	Table	*	*	implied:Table
302	Significant Accounting Policies [Abstract]	Abstract			bt:SignificantAccountingPoliciesAbstract
303	Significant accounting policies	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfSummaryOfSignificantAccountingPoliciesExplanatory
304	100150 - Disclosure - Segment information	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureSegmentInformation
305	Implied Table	Table	*	*	implied:Table
306	Disclosure Of Operating Segments [Abstract]	Abstract			ifrs-full:DisclosureOfOperatingSegmentsAbstract
307	Segment information	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfEntitysReportableSegmentsExplanatory
308	100160 - Disclosure - Operating costs	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureOperatingCosts
309	Implied Table	Table	*	*	implied:Table
310	Operating Costs [Abstract]	Abstract			bt:OperatingCostsAbstract
311	Operating costs	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfExpensesExplanatory
312	100170 - Disclosure - Employees	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureEmployees
313	Implied Table	Table	*	*	implied:Table
314	Employees [Abstract]	Abstract			bt:EmployeesAbstract
315	Employees	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfInformationAboutEmployeesExplanatory
316	100180 - Disclosure - Audit, audit related and other non-audit services	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureAuditAuditRelatedAndOtherNonAuditServices
317	Implied Table	Table	*	*	implied:Table
318	Non Audit Fees [Abstract]	Abstract			bt:NonAuditFeesAbstract
319	Audit, audit related and other non-audit services	Concept (Text Block (HTML))	For Period		bt:DisclosureOfAuditAuditRelatedAndOtherNonauditServicesExplanatory
320	100190 - Disclosure - Specific items	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureSpecificItems
321	Implied Table	Table	*	*	implied:Table
322	Specific Items [Abstract]	Abstract			bt:SpecificItemsAbstract
323	Specific items	Concept (Text Block (HTML))	For Period		bt:DisclosureOfSpecificItemsExplanatory
324	100200 - Disclosure - Taxation	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTaxation
325	Implied Table	Table	*	*	implied:Table
326	Taxation [Abstract]	Abstract			bt:TaxationAbstract
327	Taxation	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfIncomeTaxExplanatory
328	100210 - Disclosure - Earnings per share	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureEarningsPerShare
329	Implied Table	Table	*	*	implied:Table
330	Earnings Per Share [Abstract]	Abstract			ifrs-full:EarningsPerShareAbstract
331	Earnings per share	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfEarningsPerShareExplanatory
332	100220 - Disclosure - Dividends	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureDividends
333	Implied Table	Table	*	*	implied:Table
334	Dividend [Abstract]	Abstract			bt:DividendAbstract
335	Dividends	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfDividendsExplanatory
336	100230 - Disclosure - Intangible assets	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureIntangibleAssets
337	Implied Table	Table	*	*	implied:Table
338	Intangible Assets [Abstract]	Abstract			bt:IntangibleAssetsAbstract
339	Intangible assets	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfIntangibleAssetsExplanatory
340	100240 - Disclosure - Property, plant and equipment	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosurePropertyPlantAndEquipment
341	Implied Table	Table	*	*	implied:Table
342	Disclosure Of Property Plant And Equipment [Abstract]	Abstract			ifrs-full:DisclosureOfPropertyPlantAndEquipmentAbstract
343	Property, plant and equipment	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfPropertyPlantAndEquipmentExplanatory
344	100250 - Disclosure - Business combinations	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureBusinessCombinations
345	Implied Table	Table	*	*	implied:Table
346	Disclosure Of Business Combinations [Abstract]	Abstract			ifrs-full:DisclosureOfBusinessCombinationsAbstract
347	Business combinations	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfBusinessCombinationsExplanatory
348	100260 - Disclosure - Programme rights	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureProgrammeRights
349	Implied Table	Table	*	*	implied:Table
350	Disclosure Of Programme Rights [Abstract]	Abstract			bt:DisclosureOfProgrammeRightsAbstract
351	Programme rights	Concept (Text Block (HTML))	For Period		bt:DisclosureOfProgrammeRightsExplanatory
352	100270 - Disclosure - Inventories	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureInventories
353	Implied Table	Table	*	*	implied:Table
354	Inventories [Abstract]	Abstract			bt:InventoriesAbstract
355	Inventories	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfInventoriesExplanatory
356	100280 - Disclosure - Trade and other receivables	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTradeAndOtherReceivables
357	Implied Table	Table	*	*	implied:Table
358	Disclosure Of Trade And Other Receivables [Abstract]	Abstract			bt:DisclosureOfTradeAndOtherReceivablesAbstract
359	Trade and other receivables	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfTradeAndOtherReceivablesExplanatory
360	100290 - Disclosure - Trade and other payables	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTradeAndOtherPayables
361	Implied Table	Table	*	*	implied:Table
362	Trade And Other Payables [Abstract]	Abstract			ifrs-full:TradeAndOtherPayablesAbstract
363	Trade and other payables	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfTradeAndOtherPayablesExplanatory
364	100300 - Disclosure - Provisions	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureProvisions
365	Implied Table	Table	*	*	implied:Table
366	Provisions [Abstract]	Abstract			ifrs-full:ProvisionsAbstract
367	Provisions	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfProvisionsExplanatory
368	100310 - Disclosure - Retirement Benefit Plans	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlans
369	Implied Table	Table	*	*	implied:Table
370	Disclosure Of Defined Benefit Plans [Abstract]	Abstract			ifrs-full:DisclosureOfDefinedBenefitPlansAbstract
371	Retirement Benefit Plans	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfDefinedBenefitPlansExplanatory
372	100320 - Disclosure - Own shares	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureOwnShares
373	Implied Table	Table	*	*	implied:Table
374	Text Block1 [Abstract]	Abstract			bt:TextBlock1Abstract
375	Own shares	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfTreasurySharesExplanatory
376	100330 - Disclosure - Share-based payments	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureShareBasedPayments
377	Implied Table	Table	*	*	implied:Table
378	Text Block1 [Abstract]	Abstract			bt:TextBlock1Abstract
379	Share-based payments	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfSharebasedPaymentArrangementsExplanatory
380	100340 - Disclosure - Investments	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureInvestments
381	Implied Table	Table	*	*	implied:Table
382	Disclosure Of Investments [Abstract]	Abstract			bt:DisclosureOfInvestmentsAbstract
383	Investments	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfAvailableforsaleAssetsExplanatory
384	100350 - Disclosure - Cash and cash equivalents	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureCashAndCashEquivalents
385	Implied Table	Table	*	*	implied:Table
386	Cash And Cash Equivalents [Abstract]	Abstract			ifrs-full:CashAndCashEquivalentsAbstract
387	Cash and cash equivalents	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfCashAndCashEquivalentsExplanatory
388	100360 - Disclosure - Loans and other borrowings	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureLoansAndOtherBorrowings
389	Implied Table	Table	*	*	implied:Table
390	Text Block1 [Abstract]	Abstract			bt:TextBlock1Abstract
391	Loans and other borrowings	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfBorrowingsExplanatory
392	100370 - Disclosure - Finance expense	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinanceExpense
393	Implied Table	Table	*	*	implied:Table
394	Disclosure Of Finance Income Expense [Abstract]	Abstract			bt:DisclosureOfFinanceIncomeExpenseAbstract
395	Finance expense	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfFinanceIncomeExpenseExplanatory
396	100380 - Disclosure - Financial instruments and risk management	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinancialInstrumentsAndRiskManagement
397	Implied Table	Table	*	*	implied:Table
398	Text Block1 [Abstract]	Abstract			bt:TextBlock1Abstract
399	Financial instruments and risk management	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfFinancialRiskManagementExplanatory
400	100390 - Disclosure - Other reserves	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureOtherReserves
401	Implied Table	Table	*	*	implied:Table
402	Disclosure Of Other Reserves [Abstract]	Abstract			bt:DisclosureOfOtherReservesAbstract
403	Other reserves	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfReservesAndOtherEquityInterestExplanatory
404	100400 - Disclosure - Related party transactions	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRelatedPartyTransactions
405	Implied Table	Table	*	*	implied:Table
406	Disclosure Of Transactions Between Related Parties [Abstract]	Abstract			ifrs-full:DisclosureOfTransactionsBetweenRelatedPartiesAbstract
407	Related party transactions	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfRelatedPartyExplanatory
408	100410 - Disclosure - Financial commitments and contingent liabilities	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinancialCommitmentsAndContingentLiabilities
409	Implied Table	Table	*	*	implied:Table
410	Financial Commitments [Abstract]	Abstract			bt:FinancialCommitmentsAbstract
411	Financial commitments and contingent liabilities	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfCommitmentsAndContingentLiabilitiesExplanatory
412	100420 - Disclosure - Post Balance Sheet events	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosurePostBalanceSheetEvents
413	Implied Table	Table	*	*	implied:Table
414	Post Balance Sheet Events [Abstract]	Abstract			bt:PostBalanceSheetEventsAbstract
415	Post balance sheet events	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfEventsAfterReportingPeriodExplanatory
416	100430 - Disclosure - Significant accounting policies (Policies)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureSignificantAccountingPoliciesPolicies
417	Implied Table	Table	*	*	implied:Table
418	Significant Accounting Policies [Abstract]	Abstract			bt:SignificantAccountingPoliciesAbstract
419	Basis of consolidation	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfBasisOfConsolidationExplanatory
420	Revenue	Concept (Text Block (HTML))	For Period		ifrs-full:DescriptionOfAccountingPolicyForRecognitionOfRevenue
421	Operating and reportable segments	Concept (Text Block (HTML))	For Period		bt:DescriptionOfAccountingPolicyForOperatingAndReportableSegmentsExplanatory
422	Retirement benefits	Concept (Text Block (HTML))	For Period		ifrs-full:DescriptionOfAccountingPolicyForEmployeeBenefitsExplanatory
423	Property, plant and equipment	Concept (Text Block (HTML))	For Period		ifrs-full:DescriptionOfAccountingPolicyForPropertyPlantAndEquipmentExplanatory
424	Network share assets	Concept (Text Block (HTML))	For Period		bt:DescriptionOfAccountingPoliciesForNetworkShareAssetsExplanatory
425	Intangible assets	Concept (Text Block (HTML))	For Period		ifrs-full:DescriptionOfAccountingPolicyForIntangibleAssetsAndGoodwillExplanatory
426	Inventories	Concept (Text Block (HTML))	For Period		ifrs-full:DescriptionOfAccountingPolicyForMeasuringInventories
427	Provisions	Concept (Text Block (HTML))	For Period		ifrs-full:DescriptionOfAccountingPolicyForProvisionsExplanatory
428	Current and deferred income tax	Concept (Text Block (HTML))	For Period		ifrs-full:DescriptionOfAccountingPolicyForIncomeTaxExplanatory
429	Impairment of non-financial assets	Concept (Text Block (HTML))	For Period		ifrs-full:DescriptionOfAccountingPolicyForImpairmentOfNonfinancialAssetsExplanatory
430	Government grants	Concept (Text Block (HTML))	For Period		ifrs-full:DescriptionOfAccountingPolicyForGovernmentGrants
431	Foreign currencies	Concept (Text Block (HTML))	For Period		ifrs-full:DescriptionOfAccountingPolicyForForeignCurrencyTranslationExplanatory
432	Research and development	Concept (Text Block (HTML))	For Period		ifrs-full:DescriptionOfAccountingPolicyForResearchAndDevelopmentExpenseExplanatory
433	Leases	Concept (Text Block (HTML))	For Period		ifrs-full:DescriptionOfAccountingPolicyForLeasesExplanatory
434	Own shares	Concept (Text Block (HTML))	For Period		ifrs-full:DescriptionOfAccountingPolicyForTreasurySharesExplanatory
435	Share-based payments	Concept (Text Block (HTML))	For Period		ifrs-full:DescriptionOfAccountingPolicyForSharebasedPaymentTransactionsExplanatory
436	Termination benefits	Concept (Text Block (HTML))	For Period		ifrs-full:DescriptionOfAccountingPolicyForTerminationBenefits
437	Financial instruments	Concept (Text Block (HTML))	For Period		ifrs-full:DescriptionOfAccountingPolicyForFinancialInstrumentsExplanatory
438	100440 - Disclosure - Basis of presentation (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureBasisOfPresentationTables
439	Implied Table	Table	*	*	implied:Table
440	Basis Of Preparation [Abstract]	Abstract			bt:BasisOfPreparationAbstract
441	Summary of adjustments	Concept (Text Block (HTML))	For Period		ifrs-full:DescriptionOfNatureAmountAndCorrectionOfAccountingErrorsInPriorPeriodsEstimate
442	100450 - Disclosure - Significant accounting policies (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureSignificantAccountingPoliciesTables
443	Implied Table	Table	*	*	implied:Table
444	Significant Accounting Policies [Abstract]	Abstract			bt:SignificantAccountingPoliciesAbstract
445	Summary of Estimated Useful Lives Assigned to Principal Categories of Assets	Concept (Text Block (HTML))	For Period		bt:DisclosureOfDetailedInformationAboutEstimatedUsefulLivesOfPropertyPlantAndEquipmentExplanatory
446	Summary of Estimated Useful Economic Lives Assigned to Principal Catagories of Intangible Assets	Concept (Text Block (HTML))	For Period		bt:DisclosureOfDetailedInformationAboutestimatedUseFulLivesIntangibleAssetsExplanatory
447	100460 - Disclosure - Segment information (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureSegmentInformationTables
448	Implied Table	Table	*	*	implied:Table
449	Disclosure Of Operating Segments [Abstract]	Abstract			ifrs-full:DisclosureOfOperatingSegmentsAbstract
450	Summary of Results of Reportable Segments	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfOperatingSegmentsExplanatory
451	Summary of Internal Revenue and Costs Recorded by Each Reportable Segment	Concept (Text Block (HTML))	For Period		bt:DisclosureOfInternalRevenueAndCostsRecordedByEachLineOfLineOfBusinessExplanatory
452	Summary of Revenue by Products and Services	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfProductsAndServicesExplanatory
453	Summary of Capital Expenditure	Concept (Text Block (HTML))	For Period		bt:DisclosureOfCapitalExpendituresByOperatingSegmentsExplanatory
454	Summary of Geographic Information by Revenue from External Customers and Non-Current Assets	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfGeographicalAreasExplanatory
455	100470 - Disclosure - Operating costs (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureOperatingCostsTables
456	Implied Table	Table	*	*	implied:Table
457	Operating Costs [Abstract]	Abstract			bt:OperatingCostsAbstract
458	Summary of Operating Costs	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfExpensesByNatureExplanatory
459	Summary of Compensation of Key Management Personnel	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfInformationAboutKeyManagementPersonnelExplanatory
460	100480 - Disclosure - Employees (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureEmployeesTables
461	Implied Table	Table	*	*	implied:Table
462	Employees [Abstract]	Abstract			bt:EmployeesAbstract
463	Summary of Number of Employees in the Group	Concept (Text Block (HTML))	For Period		bt:DisclosureOfNumberAndAverageNumberOfEmployees
464	100490 - Disclosure - Audit, audit related and other non-audit services (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureAuditAuditRelatedAndOtherNonAuditServicesTables
465	Implied Table	Table	*	*	implied:Table
466	Non Audit Fees [Abstract]	Abstract			bt:NonAuditFeesAbstract
467	Summary of Fees Paid or Payable to Company's Auditors	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfAuditorsRemunerationExplanatory
468	Summary of Total Fees Received by Company's Auditors from the BT Pension Scheme	Concept (Text Block (HTML))	For Period		bt:SummaryOfAuditorRemunerationInRespectOfEmployeePensionFundExplanatory
469	100500 - Disclosure - Specific items (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureSpecificItemsTables
470	Implied Table	Table	*	*	implied:Table
471	Specific Items [Abstract]	Abstract			bt:SpecificItemsAbstract
472	Summary of Specific Items	Concept (Text Block (HTML))	For Period		bt:DisclosureOfDetailInformationAboutSpecificItemsExplanatory
473	100510 - Disclosure - Taxation (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTaxationTables
474	Implied Table	Table	*	*	implied:Table
475	Taxation [Abstract]	Abstract			bt:TaxationAbstract
476	Analysis of Our Taxation Expenses	Concept (Text Block (HTML))	For Period		bt:DisclosureOfIncomeTaxExpenseBenefitsExplanatory
477	Summary of Factors Affecting Our Taxation Expense for the Year	Concept (Text Block (HTML))	For Period		bt:DisclosureOfFactorsAffectingOurTaxationExpenseForTheYearExplanatory
478	Summary of Tax Components of Other Comprehensive Income	Concept (Text Block (HTML))	For Period		bt:DisclosureOfIncomeTaxRelatingToComponentsOfOtherComprehensiveIncomeExplanatory
479	Summary of Tax (Expense) Credit Recognised Directly in Equity	Concept (Text Block (HTML))	For Period		bt:DisclosureOfTaxExpenseCreditRecognisedDirectlyInEquityExplanatory
480	Summary of Deferred Taxation	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfTemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsExplanatory
481	Summary of Restricted Loss	Concept (Text Block (HTML))	For Period		bt:DisclosureOfUnrecognisedTaxLossesAndOtherTemporaryDifferencesExplanatory
482	100520 - Disclosure - Earnings per share (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureEarningsPerShareTables
483	Implied Table	Table	*	*	implied:Table
484	Earnings Per Share [Abstract]	Abstract			ifrs-full:EarningsPerShareAbstract
485	Schedule of Calculation of the Total Diluted Number of Shares	Concept (Text Block (HTML))	For Period		ifrs-full:EarningsPerShareExplanatory
486	100530 - Disclosure - Dividends (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureDividendsTables
487	Implied Table	Table	*	*	implied:Table
488	Dividend [Abstract]	Abstract			bt:DividendAbstract
489	Summary of Final and Interim Dividend Per Share and Amount	Concept (Text Block (HTML))	For Period		bt:DisclosureOfFinalAndInterimDividendPerShareAndAmountExplanatory
490	100540 - Disclosure - Intangible assets (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureIntangibleAssetsTables
491	Implied Table	Table	*	*	implied:Table
492	Intangible Assets [Abstract]	Abstract			bt:IntangibleAssetsAbstract
493	Summary of Intangible Assets	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfReconciliationOfChangesInIntangibleAssetsAndGoodwillExplanatory
494	Summary of Goodwill Allocated to Cash Generating Units	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfGoodwillExplanatory
495	100550 - Disclosure - Property, plant and equipment (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosurePropertyPlantAndEquipmentTables
496	Implied Table	Table	*	*	implied:Table
497	Disclosure Of Property Plant And Equipment [Abstract]	Abstract			ifrs-full:DisclosureOfPropertyPlantAndEquipmentAbstract
498	Summary of the Carrying Values of Software, Property, Plant and Equipment	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfDetailedInformationAboutPropertyPlantAndEquipmentExplanatory
499	Summary of Property Plant and Equipment	Concept (Text Block (HTML))	For Period		bt:DisclosureOfInformationAboutPropertyPlantAndEquipmentExplanatory
500	100560 - Disclosure - Programme rights (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureProgrammeRightsTables
501	Implied Table	Table	*	*	implied:Table
502	Disclosure Of Programme Rights [Abstract]	Abstract			bt:DisclosureOfProgrammeRightsAbstract
503	Summary of Programme Rights	Concept (Text Block (HTML))	For Period		bt:DisclosureOfDetailedProgrammeRightsExplanatory
504	100570 - Disclosure - Inventories (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureInventoriesTables
505	Implied Table	Table	*	*	implied:Table
506	Inventories [Abstract]	Abstract			bt:InventoriesAbstract
507	Summary of Inventories	Concept (Text Block (HTML))	For Period		bt:DisclosureOfCurrentInventoriesExplanatory
508	100580 - Disclosure - Trade and other receivables (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTradeAndOtherReceivablesTables
509	Implied Table	Table	*	*	implied:Table
510	Disclosure Of Trade And Other Receivables [Abstract]	Abstract			bt:DisclosureOfTradeAndOtherReceivablesAbstract
511	Schedule of Trade and Other Receivables	Concept (Text Block (HTML))	For Period		bt:DisclosureOfDetailedInformationOfTradeAndOtherReceivablesExplanatory
512	Summary of Trade Receivables After Deducting Allowances for Doubtful Debts	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfProvisionMatrixExplanatory
513	Summary of Trade Receivables Past Due and Not Specifically Impaired	Concept (Text Block (HTML))	For Period		ifrs-full:AnalysisOfAgeOfFinancialAssetsThatArePastDueButNotImpaired
514	Schedule of Trade Receivables Not Past Due and Accrued Income	Concept (Text Block (HTML))	For Period		bt:DisclosureOfTradeReceivablesNotPastDueAndAccruedIncomeExplanatory
515	100590 - Disclosure - Trade and other payables (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTradeAndOtherPayablesTables
516	Implied Table	Table	*	*	implied:Table
517	Trade And Other Payables [Abstract]	Abstract			ifrs-full:TradeAndOtherPayablesAbstract
518	Summary of Trade and Other Payables	Concept (Text Block (HTML))	For Period		bt:DisclosureOfDetailedInformationAboutTradeAndOtherPayablesExplanatory
519	100600 - Disclosure - Provisions (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureProvisionsTables
520	Implied Table	Table	*	*	implied:Table
521	Provisions [Abstract]	Abstract			ifrs-full:ProvisionsAbstract
522	Details of Estimate of the Provisions	Concept (Text Block (HTML))	For Period		bt:DisclosureOfChangesInProvisionExplanatory
523	Details of Provisions	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfOtherProvisionsExplanatory
524	100610 - Disclosure - Retirement Benefit Plans (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlansTables
525	Disclosure Of Defined Benefit Plans [Table]	Table	*	*	ifrs-full:DisclosureOfDefinedBenefitPlansTable
526	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
527	Defined benefit plans [member]	Member			ifrs-full:DefinedBenefitPlansMember
528	BTPS [Member]	Member			bt:BTPSMember
529	Disclosure Of Defined Benefit Plans [Line Items]	LineItems			ifrs-full:DisclosureOfDefinedBenefitPlansLineItems
530	Summary of Retirement Benefit Plan	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfAdditionalInformationAboutDefinedBenefitPlansExplanatory
531	Summary of Expense or Income from Retirement Benefit Arrangements Recognised in Income Statement	Concept (Text Block (HTML))	For Period		bt:DisclosureOfExpenseOrIncomeArisingFromRetirementBenefitArrangementsExplanatory
532	Summary of Pension Obligation in Respect of Defined Benefit Plans Reported in Balance Sheet	Concept (Text Block (HTML))	For Period		bt:DisclosureOfNetPensionObligationInRespectOfDefinedBenefitPlansExplanatory
533	Summary of Movements on Pension Assets and Liabilities	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfNetDefinedBenefitLiabilityAssetExplanatory
534	Summary of Analysis of Membership	Concept (Text Block (HTML))	For Period		bt:DisclosureOfAnalysisOfMembershipExplanatory
535	Summary of Fair Value of Assets of BTPS Analysed by Asset Category	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfFairValueOfPlanAssetsExplanatory
536	Summary of Approach Used to Set Key IAS 19 Assumptions	Concept (Text Block (HTML))	For Period		bt:DisclosureOfApproachUsedToSetEmployeeBenefitsExplanatory
537	Summary of Key Financial Assumptions Used to Measure Liabilities of BTPS	Concept (Text Block (HTML))	For Period		bt:DisclosureOfKeyFinancialAssumptionsUsedToMeasureLiabilitiesOfBTPSExplanatory
538	Summary of Average Life Expectancies for Members Aged Sixty	Concept (Text Block (HTML))	For Period		bt:DisclosureOfForecastLifeExpectanciesForBtpsMembersAgedSixtyExplanatory
539	Summary of Assessed Potential Negative Impact of Key Risk	Concept (Text Block (HTML))	For Period		bt:DisclosureOfPotentialNegativeImpactOfKeyRisksExplanatory
540	Summary of Two Most Recent Triennial Valuations	Concept (Text Block (HTML))	For Period		bt:DisclosureOfTwoMostRecentTriennialValuationsExplanatory
541	Summary of Prudent Long-Term Assumptions	Concept (Text Block (HTML))	For Period		bt:DisclosureOfWeightedAverageAssumptionsUsedForActuarialValuationsExplanatory
542	Summary of Payments Made to BTPS	Concept (Text Block (HTML))	For Period		bt:DisclosureOfPaymentsMadeToTheBTPSExplanatory
543	Schedule of Future Deficit Payment to BTPS	Concept (Text Block (HTML))	For Period		bt:DisclosureOfDefinedBenefitPlanExpectedFutureContributionsExplanatory
544	100620 - Disclosure - Own shares (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureOwnSharesTables
545	Implied Table	Table	*	*	implied:Table
546	Text Block1 [Abstract]	Abstract			bt:TextBlock1Abstract
547	Summary of Own Shares	Concept (Text Block (HTML))	For Period		bt:DisclosureOfTreasuryShares
548	100630 - Disclosure - Share-based payments (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureShareBasedPaymentsTables
549	Implied Table	Table	*	*	implied:Table
550	Text Block1 [Abstract]	Abstract			bt:TextBlock1Abstract
551	Summary of Plans and Analysis of Total Charge by Type of Award	Concept (Text Block (HTML))	For Period		bt:DisclosureOfShareBasedCompensationExpenseExplanatory
552	Movements in Employee Saveshare Options	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfNumberAndWeightedAverageExercisePricesOfShareOptionsExplanatory
553	Summarises Information Relating to Options Outstanding and Exercisable Under Employee Saveshare Plans	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfRangeOfExercisePricesOfOutstandingShareOptionsExplanatory
554	Movements in Executive Share Plan Awards	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfNumberAndWeightedAverageExercisePricesOfOtherEquityInstrumentsExplanatory
555	Summary of Fair Values and Key Assumptions Used for Valuing Grants Made Under Employee Saveshare Plans and ISP	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfIndirectMeasurementOfFairValueOfGoodsOrServicesReceivedShareOptionsGrantedDuringPeriodExplanatory
556	100640 - Disclosure - Investments (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureInvestmentsTables
557	Implied Table	Table	*	*	implied:Table
558	Disclosure Of Investments [Abstract]	Abstract			bt:DisclosureOfInvestmentsAbstract
559	Summary of Investments	Concept (Text Block (HTML))	For Period		bt:DisclosureOfDetailedInformationAboutInvestmentsExplanatory
560	Fair Value Hierarchy of Investments	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfFairValueMeasurementOfAssetsExplanatory
561	100650 - Disclosure - Cash and cash equivalents (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureCashAndCashEquivalentsTables
562	Implied Table	Table	*	*	implied:Table
563	Cash And Cash Equivalents [Abstract]	Abstract			ifrs-full:CashAndCashEquivalentsAbstract
564	Summary of Cash and Cash Equivalents	Concept (Text Block (HTML))	For Period		bt:DisclosureOfDetailedInformationAboutCashAndCashEquivalentsExplanatory
565	100660 - Disclosure - Loans and other borrowings (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureLoansAndOtherBorrowingsTables
566	Implied Table	Table	*	*	implied:Table
567	Text Block1 [Abstract]	Abstract			bt:TextBlock1Abstract
568	Summary of Components of Capital	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfIssuedCapitalExplanatory
569	Summary of Reconciliation From Most Directly Comparable IFRS Measure to Net Debt	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfDetailedInformationAboutBorrowingsExplanatory
570	Summary of Key Components of Net Debt	Concept (Text Block (HTML))	For Period		bt:DisclosureOfKeyComponentsOfNetDebtExplanatory
571	Disclosure Of Loans And Borrowings Explanatory	Concept (Text Block (HTML))	For Period		bt:DisclosureOfLoansAndBorrowingsExplanatory
572	Summary of Principal Repayments of Loans and Other Borrowings	Concept (Text Block (HTML))	For Period		bt:DisclosureOfPrincipalRepaymentsOfLoansAndOtherBorrowingsExplanatory
573	Summary of Obligations Under Finance Leases	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfRecognisedFinanceLeaseAsAssetsByLesseeExplanatory
574	100670 - Disclosure - Finance expense (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinanceExpenseTables
575	Implied Table	Table	*	*	implied:Table
576	Disclosure Of Finance Income Expense [Abstract]	Abstract			bt:DisclosureOfFinanceIncomeExpenseAbstract
577	Summary of Finance Expense	Concept (Text Block (HTML))	For Period		bt:DisclosureOfDetailedInformationAboutFinanceCostAndIncomeExplanatory
578	Summary of Reconciliation of Net Finance Expense to Net Interest Cash Outflow	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfFinanceCostExplanatory
579	100680 - Disclosure - Financial instruments and risk management (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinancialInstrumentsAndRiskManagementTables
580	Implied Table	Table	*	*	implied:Table
581	Text Block1 [Abstract]	Abstract			bt:TextBlock1Abstract
582	Summary of Currency and Interest Rate Profile of Loans and Borrowings	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfFinancialInstrumentsByTypeOfInterestRateExplanatory
583	Summary of Impact on Equity, Before Tax, of a 1% Increase in Interest	Concept (Text Block (HTML))	For Period		bt:DisclosureOfSensitivityRelatesToInterestRateExplanatoryExplanatory
584	Summary of Credit Ratings	Concept (Text Block (HTML))	For Period		ifrs-full:AnalysisOfCreditExposuresUsingExternalCreditGradingSystemExplanatory
585	Remaining Contractually-Agreed Cash Flows Including Interest Payable for Non-Derivative Financial Liabilities on Undiscounted Basis	Concept (Text Block (HTML))	For Period		ifrs-full:MaturityAnalysisForNonderivativeFinancialLiabilities
586	Summary of Contractually Agreed Cash Flows in Respect of Derivative Financial Instruments	Concept (Text Block (HTML))	For Period		ifrs-full:MaturityAnalysisForDerivativeFinancialLiabilities
587	Summary of Maximum Credit Risk Exposure of Financial Assets	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfCreditRiskExposureExplanatory
588	Summary of Credit Quality and Credit Concentration of Cash Equivalents, Current Asset Investments and Derivative Financial Assets	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfExternalCreditExposuresExplanatory
589	Summary of Offsetting of Financial Assets and Liabilities	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfOffsettingOfFinancialAssetsAndFinancialLiabilitiesExplanatory
590	Summary of Derivative Financial Instruments are Held at Fair Value on Balance Sheet	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfDerivativeFinancialInstrumentsExplanatory
591	100690 - Disclosure - Other reserves (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureOtherReservesTables
592	Implied Table	Table	*	*	implied:Table
593	Disclosure Of Reserves Within Equity [Abstract]	Abstract			ifrs-full:DisclosureOfReservesWithinEquityAbstract
594	Summary of Other Reserves	Concept (Text Block (HTML))	For Period		bt:DisclosureOfDetailedInformationAboutOtherReservesExplanatory
595	100700 - Disclosure - Financial commitments and contingent liabilities (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinancialCommitmentsAndContingentLiabilitiesTables
596	Implied Table	Table	*	*	implied:Table
597	Financial Commitments [Abstract]	Abstract			bt:FinancialCommitmentsAbstract
598	Disclosure of Financial Commitments	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfCommitmentsExplanatory
599	Disclosure of Future Minimum Operating Lease Payments	Concept (Text Block (HTML))	For Period		bt:DisclosureOfOperatingLeasesFutureMinimumPaymentsExplanatory
600	100710 - Disclosure - Post Balance Sheet events (Tables)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosurePostBalanceSheetEventsTables
601	Disclosure Of Nonadjusting Events After Reporting Period [Table]	Table	*	*	ifrs-full:DisclosureOfNonadjustingEventsAfterReportingPeriodTable
602	Non-adjusting events after reporting period [axis]	Axis			ifrs-full:NonadjustingEventsAfterReportingPeriodAxis
603	Non-adjusting events after reporting period [member]	Member			ifrs-full:NonadjustingEventsMember
604	Segment Revenue and Profit [Member]	Member			bt:SegmentRevenueAndProfitMember
605	Internal Revenue and Costs [Member]	Member			bt:InternalRevenueAndCostsMember
606	Capital Expenditure [Member]	Member			bt:CapitalExpenditureMember
607	Disclosure Of Nonadjusting Events After Reporting Period [Line Items]	LineItems			ifrs-full:DisclosureOfNonadjustingEventsAfterReportingPeriodLineItems
608	Summary of Results of Reportable Segments	Concept (Text Block (HTML))	For Period		ifrs-full:DisclosureOfOperatingSegmentsExplanatory
609	Summary of Internal Revenue and Costs Recorded by Each Reportable Segment	Concept (Text Block (HTML))	For Period		bt:DisclosureOfInternalRevenueAndCostsRecordedByEachLineOfLineOfBusinessExplanatory
610	Summary of Capital Expenditure	Concept (Text Block (HTML))	For Period		bt:DisclosureOfCapitalExpendituresByOperatingSegmentsExplanatory
611	100720 - Disclosure - Basis of Preparation - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureBasisOfPreparationAdditionalInformationDetail
612	Disclosure Of Basis Of Preparation Of Financial Statements [Table]	Table	*	*	bt:DisclosureOfBasisOfPreparationOfFinancialStatementsTable
613	Components of equity [axis]	Axis			ifrs-full:ComponentsOfEquityAxis
614	Equity [member]	Member			ifrs-full:EquityMember
615	Retained (Loss) Earnings [Member]	Member			ifrs-full:RetainedEarningsMember
616	Range [axis]	Axis			ifrs-full:RangeAxis
617	Ranges [member]	Member			ifrs-full:RangesMember
618	Bottom of Range [Member]	Member			ifrs-full:BottomOfRangeMember
619	Top of Range [Member]	Member			ifrs-full:TopOfRangeMember
620	Non-adjusting events after reporting period [axis]	Axis			ifrs-full:NonadjustingEventsAfterReportingPeriodAxis
621	Non-adjusting events after reporting period [member]	Member			ifrs-full:NonadjustingEventsMember
622	Financial Impact [Member]	Member			bt:FinancialImpactMember
623	Retrospective application and retrospective restatement [axis]	Axis			ifrs-full:RetrospectiveApplicationAndRetrospectiveRestatementAxis
624	Currently stated [member]	Member			ifrs-full:RestatedMember
625	Increase (decrease) due to corrections of prior period errors [member]	Member			ifrs-full:FinancialEffectOfCorrectionsOfAccountingErrorsMember
626	Disclosure Of Basis Of Preparation Of Financial Statements [Line Items]	LineItems			bt:DisclosureOfBasisOfPreparationOfFinancialStatementsLineItems
627	Cumulative increase in retained earnings	Concept (Monetary)	For Period		ifrs-full:IncreaseDecreaseInAccountingEstimate
628	Net defined benefit obligation after tax	Concept (Monetary)	As Of	Credit	bt:NetDefinedBenefitObligationAfterTax
629	Increase in retirement benefit obligation,gross	Concept (Monetary)	As Of	Credit	bt:NetDefinedBenefitObligationGross
630	Total equity	Concept (Monetary)	As Of	Credit	ifrs-full:Equity
631	Remeasurements of the net pension obligation	Concept (Monetary)	For Period	Credit	ifrs-full:OtherComprehensiveIncomeBeforeTaxGainsLossesOnRemeasurementsOfDefinedBenefitPlans
632	Tax expense on pension re-measurement	Concept (Monetary)	For Period	Debit	ifrs-full:IncomeTaxRelatingToRemeasurementsOfDefinedBenefitPlansOfOtherComprehensiveIncome
633	100730 - Disclosure - Basis of Preparation - Group statement of comprehensive income	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureBasisOfPreparationGroupStatementOfComprehensiveIncome
634	Statements [Table]	Table	*	*	bt:StatementsTable
635	Retrospective application and retrospective restatement [axis]	Axis			ifrs-full:RetrospectiveApplicationAndRetrospectiveRestatementAxis
636	Currently stated [member]	Member			ifrs-full:RestatedMember
637	As Published [Member]	Member			ifrs-full:PreviouslyStatedMember
638	Pension Adjustment [Member]	Member			bt:PensionAdjustmentMember
639	Statements [Line Items]	LineItems			bt:StatementsLineItems
640	Profit for the year	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLoss
641	Other comprehensive income (loss)	Abstract			ifrs-full:OtherComprehensiveIncomeAbstract
642	Items that will not be reclassified to the income statement	Abstract			ifrs-full:ComponentsOfOtherComprehensiveIncomeThatWillNotBeReclassifiedToProfitOrLossBeforeTaxAbstract
643	Remeasurements of the net pension obligation	Concept (Monetary)	For Period	Credit	ifrs-full:OtherComprehensiveIncomeBeforeTaxGainsLossesOnRemeasurementsOfDefinedBenefitPlans
644	Tax on pension remeasurements	Concept (Monetary)	For Period	Debit	ifrs-full:IncomeTaxRelatingToRemeasurementsOfDefinedBenefitPlansOfOtherComprehensiveIncome
645	Items that have been or may be reclassified to the income statement	Abstract			ifrs-full:ComponentsOfOtherComprehensiveIncomeThatWillBeReclassifiedToProfitOrLossBeforeTaxAbstract
646	Exchange differences on translation of foreign operations	Concept (Monetary)	For Period	Credit	ifrs-full:OtherComprehensiveIncomeBeforeTaxExchangeDifferencesOnTranslation
647	Fair value movements on available-for-sale assets	Concept (Monetary)	For Period	Credit	ifrs-full:OtherComprehensiveIncomeBeforeTaxAvailableforsaleFinancialAssets
648	Fair value movements on cash flow hedges:	Abstract			ifrs-full:CashFlowHedgesAbstract
649	� net fair value gains (losses)	Concept (Monetary)	For Period	Credit	ifrs-full:OtherComprehensiveIncomeBeforeTaxCashFlowHedges
650	� recognised in income and expense	Concept (Monetary)	For Period	Credit	ifrs-full:GainsLossesOnCashFlowHedgesBeforeTax
651	Tax on components of other comprehensive income that have been or may be reclassified	Concept (Monetary)	For Period	Debit	ifrs-full:IncomeTaxRelatingToComponentsOfOtherComprehensiveIncomeThatWillBeReclassifiedToProfitOrLoss
652	Other comprehensive income (loss) for the year, net of tax	Concept (Monetary)	For Period	Credit	ifrs-full:OtherComprehensiveIncome
653	Total comprehensive income (loss) for the year	Concept (Monetary)	For Period	Credit	ifrs-full:ComprehensiveIncome
654	100740 - Disclosure - Basis of Preparation - Group balance sheet	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureBasisOfPreparationGroupBalanceSheet
655	Statements [Table]	Table	*	*	bt:StatementsTable
656	Retrospective application and retrospective restatement [axis]	Axis			ifrs-full:RetrospectiveApplicationAndRetrospectiveRestatementAxis
657	Currently stated [member]	Member			ifrs-full:RestatedMember
658	As Published [Member]	Member			ifrs-full:PreviouslyStatedMember
659	Pension Adjustment [Member]	Member			bt:PensionAdjustmentMember
660	Statements [Line Items]	LineItems			bt:StatementsLineItems
661	Non-current assets	Abstract			ifrs-full:NoncurrentAssetsAbstract
662	Deferred tax assets	Concept (Monetary)	As Of	Debit	ifrs-full:DeferredTaxAssets
663	Other non-current assets	Concept (Monetary)	As Of	Debit	ifrs-full:OtherNoncurrentAssets
664	Non current assets	Concept (Monetary)	As Of	Debit	ifrs-full:NoncurrentAssets
665	Current assets	Abstract			ifrs-full:CurrentAssetsAbstract
666	Current assets	Concept (Monetary)	As Of	Debit	ifrs-full:CurrentAssets
667	Current liabilities	Abstract			ifrs-full:CurrentLiabilitiesAbstract
668	Current liabilities	Concept (Monetary)	As Of	Credit	ifrs-full:CurrentLiabilities
669	Total assets less current liabilities	Concept (Monetary)	As Of	Debit	ifrs-full:AssetsLessCurrentLiabilities
670	Non-current liabilities	Abstract			ifrs-full:NoncurrentLiabilitiesAbstract
671	Retirement benefit obligations	Concept (Monetary)	As Of	Credit	ifrs-full:NoncurrentRecognisedLiabilitiesDefinedBenefitPlan
672	Other non-current liabilities	Concept (Monetary)	As Of	Credit	ifrs-full:OtherNoncurrentLiabilities
673	Non current liabilities	Concept (Monetary)	As Of	Credit	ifrs-full:NoncurrentLiabilities
674	Equity	Abstract			ifrs-full:EquityAbstract
675	Retained profit	Concept (Monetary)	As Of	Credit	ifrs-full:RetainedEarnings
676	Other components of equity	Concept (Monetary)	As Of	Credit	bt:OtherComponentsOfEquity
677	Total equity	Concept (Monetary)	As Of	Credit	ifrs-full:Equity
678	Total equity and Liabilities	Concept (Monetary)	As Of	Credit	ifrs-full:EquityAndLiabilities
679	100750 - Disclosure - Basis of Preparation - Group statement of changes in equity (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureBasisOfPreparationGroupStatementOfChangesInEquityDetail
680	Statements [Table]	Table	*	*	bt:StatementsTable
681	Retrospective application and retrospective restatement [axis]	Axis			ifrs-full:RetrospectiveApplicationAndRetrospectiveRestatementAxis
682	Currently stated [member]	Member			ifrs-full:RestatedMember
683	Equity Published [Member]	Member			ifrs-full:PreviouslyStatedMember
684	Retained Earnings Pension Adjustment [Member]	Member			bt:RetainedEarningsPensionAdjustmentMember
685	Statements [Line Items]	LineItems			bt:StatementsLineItems
686	Beginning balance	Concept (Monetary)	As Of	Credit	ifrs-full:Equity
687	Profit for the year	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLoss
688	Other comprehensive income (loss) � before tax	Concept (Monetary)	For Period	Credit	ifrs-full:OtherComprehensiveIncomeBeforeTax
689	Tax on other comprehensive income (loss)	Concept (Monetary)	For Period		ifrs-full:IncomeTaxRelatingToComponentsOfOtherComprehensiveIncome
690	Transferred to the income statement	Concept (Monetary)	For Period	Credit	ifrs-full:GainsLossesOnCashFlowHedgesBeforeTax
691	Comprehensive income (loss)	Concept (Monetary)	For Period	Credit	ifrs-full:ComprehensiveIncome
692	Dividends to shareholders	Concept (Monetary)	For Period	Debit	ifrs-full:DividendsPaid
693	Share-based payments	Concept (Monetary)	For Period	Credit	ifrs-full:IncreaseDecreaseThroughSharebasedPaymentTransactions
694	Tax on share-based payments	Concept (Monetary)	For Period	Debit	ifrs-full:DecreaseIncreaseThroughTaxOnSharebasedPaymentTransactions
695	Net buyback of own shares	Concept (Monetary)	For Period	Debit	ifrs-full:PurchaseOfTreasuryShares
696	Transfer to realised profit	Concept (Monetary)	For Period	Debit	bt:TransfersToRealisedProfit
697	Ending balance	Concept (Monetary)	As Of	Credit	ifrs-full:Equity
698	100760 - Disclosure - Significant Accounting Policies - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureSignificantAccountingPoliciesAdditionalInformationDetail
699	Significant Accounting Policies [Table]	Table	*	*	bt:SignificantAccountingPoliciesTable
700	Classes of assets [axis]	Axis			ifrs-full:ClassesOfAssetsAxis
701	Assets [member]	Member			ifrs-full:ClassesOfAssetsMember
702	Current Assets and Current Liabilities [Member]	Member			bt:CurrentAssetsAndCurrentLiabilitiesMember
703	Non-Current Assets or Non-Current Liabilities [Member]	Member			bt:NonCurrentAssetsOrNonCurrentLiabilitiesMember
704	Significant Accounting Policies [Line Items]	LineItems			bt:SignificantAccountingPoliciesLineItems
705	Number of operating segments	Concept (Integer)	For Period		bt:NumberOfConsumerOperatingSegments
706	Cash and cash equivalents original maturity period	Concept (Text/String)	For Period		bt:CashAndCashEquivalentOriginalMaturityPeriod
707	Derivative financial instruments original maturity period	Concept (Text/String)	For Period		bt:DerivativeFinancialInstrumentsOriginalMaturityPeriod
708	100770 - Disclosure - Significant Accounting Policies - Summary of Estimated Useful Lives Assigned to Principal Categories of Assets (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureSignificantAccountingPoliciesSummaryOfEstimatedUsefulLivesAssignedToPrincipalCategoriesOfAssetsDetail
709	Disclosure Of Property Plant And Equipment [Table]	Table	*	*	ifrs-full:DisclosureOfPropertyPlantAndEquipmentTable
710	Classes of property, plant and equipment [axis]	Axis			ifrs-full:ClassesOfPropertyPlantAndEquipmentAxis
711	Property, plant and equipment [member]	Member			ifrs-full:PropertyPlantAndEquipmentMember
712	Freehold Buildings [Member]	Member			bt:FreeholdBuildingsMember
713	Short-Term Leasehold Improvements [Member]	Member			ifrs-full:LeaseholdImprovementsMember
714	Leasehold [Member]	Member			bt:LeaseholdLandAndBuildingsMember
715	Network Infrastructure [Member]	Member			ifrs-full:NetworkInfrastructureMember
716	Motor Vehicles [Member]	Member			ifrs-full:MotorVehiclesMember
717	Computers and Office Equipment [Member]	Member			bt:ComputersAndOfficeEquipmentMember
718	Classes of assets [axis]	Axis			ifrs-full:ClassesOfAssetsAxis
719	Assets [member]	Member			ifrs-full:ClassesOfAssetsMember
720	Other Assets [Member]	Member			ifrs-full:OtherAssetsMember
721	Types of investment property [axis]	Axis			ifrs-full:TypesOfInvestmentPropertyAxis
722	Investment property [member]	Member			ifrs-full:InvestmentPropertyMember
723	Duct [Member]	Member			bt:DuctMember
724	Cable [Member]	Member			bt:CableMember
725	Fibre Member]	Member			bt:FibreMember
726	Exchange Equipment [Member]	Member			bt:ExchangeEquipmentMember
727	Other Network Equipment [Member]	Member			bt:OtherNetworkEquipmentMember
728	Disclosure of detailed information about property, plant and equipment [Line Items]	LineItems			ifrs-full:DisclosureOfPropertyPlantAndEquipmentLineItems
729	Estimated useful lives of property, plant and equipment	Concept (Text/String)	For Period		ifrs-full:UsefulLivesOrDepreciationRatesPropertyPlantAndEquipment
730	100780 - Disclosure - Significant Accounting Policies - Summary of Estimated Useful Economic Lives Assigned to Principal Categories of Intangible Assets (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureSignificantAccountingPoliciesSummaryOfEstimatedUsefulEconomicLivesAssignedToPrincipalCategoriesOfIntangibleAssetsDetail
731	Disclosure Of Intangible Assets [Table]	Table	*	*	ifrs-full:DisclosureOfIntangibleAssetsTable
732	Classes of intangible assets other than goodwill [axis]	Axis			ifrs-full:ClassesOfIntangibleAssetsOtherThanGoodwillAxis
733	Intangible assets other than goodwill [member]	Member			ifrs-full:IntangibleAssetsOtherThanGoodwillMember
734	Internally Developed Software [Member]	Member			ifrs-full:ComputerSoftwareMember
735	Telecommunications Licences [Member]	Member			bt:TelecommunicationsLicencesMember
736	Customer Relationships and Brands [Member]	Member			bt:CustomerRelationshipsAndBrandsMember
737	Disclosure of detailed information about intangible assets [Line Items]	LineItems			ifrs-full:DisclosureOfIntangibleAssetsLineItems
738	Estimated useful economic lives of intangible assets	Concept (Text/String)	For Period		ifrs-full:UsefulLivesOrAmortisationRatesIntangibleAssetsOtherThanGoodwill
739	100790 - Disclosure - Segment Information - Summary of Results of Reportable Segments (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureSegmentInformationSummaryOfResultsOfReportableSegmentsDetail
740	Disclosure Of Operating Segments [Table]	Table	*	*	ifrs-full:DisclosureOfOperatingSegmentsTable
741	Segments [axis]	Axis			ifrs-full:SegmentsAxis
742	Segments [member]	Member			ifrs-full:SegmentsMember
743	BT Consumer [Member]	Member			bt:BTConsumerMember
744	EE [Member]	Member			bt:EEMember
745	Business and Public Sector [Member]	Member			bt:BusinessAndPublicSectorMember
746	Global Services [Member]	Member			bt:GlobalServicesMember
747	Wholesale and Ventures [Member]	Member			bt:WholesaleAndVenturesMember
748	Openreach [Member]	Member			bt:OpenreachMember
749	Other [Member]	Member			ifrs-full:AllOtherSegmentsMember
750	Segment consolidation items [axis]	Axis			ifrs-full:SegmentConsolidationItemsAxis
751	Entity's total for segment consolidation items [member]	Member			ifrs-full:EntitysTotalForSegmentConsolidationItemsMember
752	Segment Revenue [Member]	Member			ifrs-full:OperatingSegmentsMember
753	Elimination of Intersegment Amounts [Member]	Member			ifrs-full:EliminationOfIntersegmentAmountsMember
754	Retrospective application and retrospective restatement [axis]	Axis			ifrs-full:RetrospectiveApplicationAndRetrospectiveRestatementAxis
755	Currently stated [member]	Member			ifrs-full:RestatedMember
756	Before Specific Items [Member]	Member			bt:BeforeSpecificItemsMember
757	Specific Items [Member]	Member			bt:SpecificItemsMember
758	Disclosure of operating segments [Line Items]	LineItems			ifrs-full:DisclosureOfOperatingSegmentsLineItems
759	Revenue	Concept (Monetary)	For Period	Credit	ifrs-full:Revenue
760	EBITDA	Concept (Monetary)	For Period	Credit	bt:EarningsBeforeInterestTaxDepreciationAndAmortization
761	Depreciation and amortisation	Concept (Monetary)	For Period	Debit	ifrs-full:DepreciationAndAmortisationExpense
762	Operating profit (loss)	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLossFromOperatingActivities
763	Net finance expense	Concept (Monetary)	For Period	Credit	ifrs-full:FinanceIncomeCost
764	Share of post tax profit (loss) of associates and joint ventures	Concept (Monetary)	For Period	Credit	ifrs-full:ShareOfOtherComprehensiveIncomeOfAssociatesAndJointVenturesAccountedForUsingEquityMethodThatWillBeReclassifiedToProfitOrLossBeforeTax
765	Profit (loss) before taxation	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLossBeforeTax
766	100800 - Disclosure - Segment Information - Summary of Results of Reportable Segments (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureSegmentInformationSummaryOfResultsOfReportableSegmentsParentheticalDetail
767	Disclosure Of Operating Segments [Table]	Table	*	*	ifrs-full:DisclosureOfOperatingSegmentsTable
768	Retrospective application and retrospective restatement [axis]	Axis			ifrs-full:RetrospectiveApplicationAndRetrospectiveRestatementAxis
769	Currently stated [member]	Member			ifrs-full:RestatedMember
770	Specific Items [Member]	Member			bt:SpecificItemsMember
771	Disclosure of operating segments [Line Items]	LineItems			ifrs-full:DisclosureOfOperatingSegmentsLineItems
772	Net finance expense	Concept (Monetary)	For Period	Credit	ifrs-full:FinanceIncomeCost
773	100810 - Disclosure - Segment Information - Summary of Internal Revenue and Costs Recorded by Each Reportable Segment (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureSegmentInformationSummaryOfInternalRevenueAndCostsRecordedByEachReportableSegmentDetail
774	Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Table]	Table	*	*	ifrs-full:DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable
775	Segment consolidation items [axis]	Axis			ifrs-full:SegmentConsolidationItemsAxis
776	Entity's total for segment consolidation items [member]	Member			ifrs-full:EntitysTotalForSegmentConsolidationItemsMember
777	Elimination of Intersegment Amounts [Member]	Member			ifrs-full:EliminationOfIntersegmentAmountsMember
778	Segments [axis]	Axis			ifrs-full:SegmentsAxis
779	Segments [member]	Member			ifrs-full:SegmentsMember
780	BT Consumer [Member]	Member			bt:BTConsumerMember
781	EE [Member]	Member			bt:EEMember
782	Business and Public Sector [Member]	Member			bt:BusinessAndPublicSectorMember
783	Global Services [Member]	Member			bt:GlobalServicesMember
784	Wholesale and Ventures [Member]	Member			bt:WholesaleAndVenturesMember
785	Openreach [Member]	Member			bt:Openreach1Member
786	Other [Member]	Member			bt:OtherSegmentMember
787	BT Consumer [Member]	Member			bt:BTConsumer1Member
788	EE [Member]	Member			bt:EELimitedMember
789	Business and Public Sector [Member]	Member			bt:BusinessAndPublicSectorsMember
790	Global Services [Member]	Member			bt:GlobalServices1Member
791	Wholesale and Ventures [Member]	Member			bt:WholesaleAndVentures1Member
792	Segment With Whom Transaction Is With	Axis			bt:SegmentWithWhomTransactionIsWithAxis
793	Segment With Whom Transaction Is With	Member			bt:SegmentWithWhomTransactionIsWithDomain
794	BT Consumer [Member]	Member			bt:BTConsumer1Member
795	EE [Member]	Member			bt:EELimitedMember
796	Business and Public Sector [Member]	Member			bt:BusinessAndPublicSectorsMember
797	Global Services [Member]	Member			bt:GlobalServices1Member
798	Wholesale and Ventures [Member]	Member			bt:WholesaleAndVentures1Member
799	Openreach [Member]	Member			bt:OpenreachMember
800	BT Consumer [Member]	Member			bt:BTConsumerMember
801	EE [Member]	Member			bt:EEMember
802	Business and Public Sector [Member]	Member			bt:BusinessAndPublicSectorMember
803	Global Services [Member]	Member			bt:GlobalServicesMember
804	Wholesale and Ventures [Member]	Member			bt:WholesaleAndVenturesMember
805	Other [Member]	Member			bt:OtherSegmentMember
806	Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]	LineItems			ifrs-full:DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems
807	Internal revenue recorded by	Abstract			bt:InternalRevenueRecordedByAbstract
808	Internal cost	Concept (Monetary)	For Period	Debit	ifrs-full:OperatingExpense
809	Internal revenue	Concept (Monetary)	For Period	Credit	ifrs-full:Revenue
810	100820 - Disclosure - Segment Information - Summary of Revenue by Products and Services (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureSegmentInformationSummaryOfRevenueByProductsAndServicesDetail
811	Disclosure Of Products And Services [Table]	Table	*	*	ifrs-full:DisclosureOfProductsAndServicesTable
812	Classes of property, plant and equipment [axis]	Axis			ifrs-full:ClassesOfPropertyPlantAndEquipmentAxis
813	Property, plant and equipment [member]	Member			ifrs-full:PropertyPlantAndEquipmentMember
814	Products and Services [Member]	Member			bt:ProductAndServiceMember
815	Products and services [axis]	Axis			ifrs-full:ProductsAndServicesAxis
816	Products and services [member]	Member			ifrs-full:ProductsAndServicesMember
817	ICT and Managed Networks [Member]	Member			bt:ICTAndManagedNetworksMember
818	Broadband and TV [Member]	Member			bt:BroadbandAndTVMember
819	Mobile [Member]	Member			bt:MobileMember
820	Calls, Lines and Connections [Member]	Member			bt:CallsLinesAndConnectionsMember
821	Transit [Member]	Member			bt:TransitMember
822	Other Products and Services [Member]	Member			bt:OtherProductsAndServicesMember
823	Disclosure of products and services [Line Items]	LineItems			ifrs-full:DisclosureOfProductsAndServicesLineItems
824	Revenue	Concept (Monetary)	For Period	Credit	ifrs-full:Revenue
825	100830 - Disclosure - Segment Information - Summary of Capital Expenditure (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureSegmentInformationSummaryOfCapitalExpenditureDetail
826	Disclosure Of Operating Segments [Table]	Table	*	*	ifrs-full:DisclosureOfOperatingSegmentsTable
827	Segments [axis]	Axis			ifrs-full:SegmentsAxis
828	Segments [member]	Member			ifrs-full:SegmentsMember
829	BT Consumer [Member]	Member			bt:BTConsumerMember
830	EE [Member]	Member			bt:EEMember
831	Business and Public Sector [Member]	Member			bt:BusinessAndPublicSectorMember
832	Global Services [Member]	Member			bt:GlobalServicesMember
833	Wholesale and Ventures [Member]	Member			bt:WholesaleAndVenturesMember
834	Openreach [Member]	Member			bt:OpenreachMember
835	Other [Member]	Member			ifrs-full:AllOtherSegmentsMember
836	Classes of intangible assets other than goodwill [axis]	Axis			ifrs-full:ClassesOfIntangibleAssetsOtherThanGoodwillAxis
837	Intangible assets other than goodwill [member]	Member			ifrs-full:IntangibleAssetsOtherThanGoodwillMember
838	Intangible Assets [Member]	Member			bt:IntangibleAssetsExcludingGoodwillMember
839	Classes of property, plant and equipment [axis]	Axis			ifrs-full:ClassesOfPropertyPlantAndEquipmentAxis
840	Property, plant and equipment [member]	Member			ifrs-full:PropertyPlantAndEquipmentMember
841	Property, Plant and Equipment [Member]	Member			bt:PropertyPlantAndEquipmentsMember
842	Disclosure of operating segments [Line Items]	LineItems			ifrs-full:DisclosureOfOperatingSegmentsLineItems
843	Capital expenditure	Concept (Monetary)	For Period	Debit	bt:CapitalExpenditure
844	Capital expenditure	Concept (Monetary)	For Period	Credit	ifrs-full:PurchaseOfPropertyPlantAndEquipmentIntangibleAssetsOtherThanGoodwillInvestmentPropertyAndOtherNoncurrentAssets
845	100840 - Disclosure - Segment Information - Summary of Geographic Information by Revenue from External Customers and Non-Current Assets (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureSegmentInformationSummaryOfGeographicInformationByRevenueFromExternalCustomersAndNonCurrentAssetsDetail
846	Disclosure Of Geographical Areas [Table]	Table	*	*	ifrs-full:DisclosureOfGeographicalAreasTable
847	Geographical areas [axis]	Axis			ifrs-full:GeographicalAreasAxis
848	Geographical areas [member]	Member			ifrs-full:GeographicalAreasMember
849	UK [Member]	Member			country:GB
850	Europe, Middle East and Africa, Excluding the UK [Member]	Member			bt:EuropeMiddleEastAndAfricaExcludingUKMember
851	Asia Pacific [Member]	Member			bt:AsiaPacific1Member
852	Americas [Member]	Member			country:US
853	Retrospective application and retrospective restatement [axis]	Axis			ifrs-full:RetrospectiveApplicationAndRetrospectiveRestatementAxis
854	Currently stated [member]	Member			ifrs-full:RestatedMember
855	Before Specific Items [Member]	Member			bt:BeforeSpecificItemsMember
856	Disclosure of geographical areas [Line Items]	LineItems			ifrs-full:DisclosureOfGeographicalAreasLineItems
857	Revenue	Concept (Monetary)	For Period	Credit	ifrs-full:Revenue
858	Non-current assets	Concept (Monetary)	As Of	Debit	bt:NoncurrentAssetsExcludeDerivativeFinancialInstrumentsDeferredTaxAssetsOrInvestments
859	100850 - Disclosure - Operating Costs - Summary of Operating Costs (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureOperatingCostsSummaryOfOperatingCostsDetail
860	Disclosure Of Operating Costs [Table]	Table	*	*	bt:DisclosureOfOperatingCostsTable
861	Classes of property, plant and equipment [axis]	Axis			ifrs-full:ClassesOfPropertyPlantAndEquipmentAxis
862	Property, plant and equipment [member]	Member			ifrs-full:PropertyPlantAndEquipmentMember
863	Owned Property Plant and Equipment [Member]	Member			bt:OwnedPropertyPlantAndEquipmentMember
864	Finace Leased Property Plant and Equipment [Member]	Member			bt:FinanceLeasedPropertyPlantAndEquipmentMember
865	Retrospective application and retrospective restatement [axis]	Axis			ifrs-full:RetrospectiveApplicationAndRetrospectiveRestatementAxis
866	Currently stated [member]	Member			ifrs-full:RestatedMember
867	Before Specific Items [Member]	Member			bt:BeforeSpecificItemsMember
868	Specific Items [Member]	Member			bt:SpecificItemsMember
869	Disclosure Of Operating Costs [Line Items]	LineItems			bt:DisclosureOfOperatingCostsLineItems
870	Wages and salaries	Concept (Monetary)	For Period	Debit	ifrs-full:WagesAndSalaries
871	Social security costs	Concept (Monetary)	For Period	Debit	ifrs-full:SocialSecurityContributions
872	Other pension costs	Concept (Monetary)	For Period	Debit	bt:OtherPensionCosts
873	Share-based payment expense	Concept (Monetary)	For Period	Debit	bt:ShareBasedPaymentExpense
874	Total staff costs	Concept (Monetary)	For Period	Debit	bt:StaffCosts
875	Own work capitalised	Concept (Monetary)	For Period	Debit	bt:OwnWorkCapitalised
876	Net staff costs	Concept (Monetary)	For Period	Debit	bt:NetStaffCosts
877	Net indirect labour costs	Concept (Monetary)	For Period	Debit	bt:NetIndirectLabourCosts
878	Net labour costs	Concept (Monetary)	For Period	Debit	bt:EmployeeCosts
879	Payments to telecommunications operators	Concept (Monetary)	For Period	Debit	bt:PaymentsToTelecommunicationsOperators
880	Property and energy costs	Concept (Monetary)	For Period	Debit	bt:PropertyAndEnergyCosts
881	Network operating and IT costs	Concept (Monetary)	For Period	Debit	bt:NetworkOperatingAndITCosts
882	TV programme rights charges	Concept (Monetary)	For Period	Debit	bt:TVProgrammeRightsCharges
883	Other operating costs	Concept (Monetary)	For Period	Debit	ifrs-full:MiscellaneousOtherOperatingExpense
884	Other operating income	Concept (Monetary)	For Period	Credit	ifrs-full:MiscellaneousOtherOperatingIncome
885	Depreciation of property, plant and equipment	Concept (Monetary)	For Period	Debit	ifrs-full:DepreciationExpense
886	Amortisation of intangible assets	Concept (Monetary)	For Period		ifrs-full:AmortisationIntangibleAssetsOtherThanGoodwill
887	Total operating costs	Concept (Monetary)	For Period	Debit	ifrs-full:OperatingExpense
888	Operating costs before specific items include the following:	Abstract			bt:OperatingCostsBeforeSpecificItemsAbstract
889	Leaver costs	Concept (Monetary)	For Period	Debit	bt:LeaverCosts
890	Research and development expenditure	Concept (Monetary)	For Period	Debit	ifrs-full:ResearchAndDevelopmentExpense
891	Operating lease charges	Concept (Monetary)	For Period	Credit	ifrs-full:OperatingLeaseIncome
892	Foreign currency gains	Concept (Monetary)	For Period	Credit	ifrs-full:GainsLossesOnExchangeDifferencesOnTranslationBeforeTax
893	Government grants	Concept (Monetary)	As Of	Credit	ifrs-full:GovernmentGrants
894	100860 - Disclosure - Operating Costs - Summary of Operating Costs (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureOperatingCostsSummaryOfOperatingCostsParentheticalDetail
895	Disclosure Of Operating Costs [Table]	Table	*	*	bt:DisclosureOfOperatingCostsTable
896	Classes of intangible assets other than goodwill [axis]	Axis			ifrs-full:ClassesOfIntangibleAssetsOtherThanGoodwillAxis
897	Intangible assets other than goodwill [member]	Member			ifrs-full:IntangibleAssetsOtherThanGoodwillMember
898	Internally Developed Software [Member]	Member			ifrs-full:ComputerSoftwareMember
899	Capitalised Software Development Costs [Member]	Member			ifrs-full:CapitalisedDevelopmentExpenditureMember
900	Methods of generation [axis]	Axis			ifrs-full:MethodsOfGenerationAxis
901	Methods of generation [member]	Member			ifrs-full:MethodsOfGenerationMember
902	Internally Developed Computer Software and Operating Expenses [Member]	Member			ifrs-full:InternallyGeneratedMember
903	Disclosure Of Operating Costs [Line Items]	LineItems			bt:DisclosureOfOperatingCostsLineItems
904	Net of capitalised indirect labour costs	Concept (Monetary)	For Period	Debit	bt:NetOfCapitalisedIndirectLabourCosts
905	Amortisation relating to write off of software costs	Concept (Monetary)	For Period	Debit	bt:AmortisationChargeRelatingToWriteOffOfSoftwareCosts
906	Leaver costs associated with restructuring and integration	Concept (Monetary)	For Period	Debit	bt:LeaverCostsRelatedToRestructuringAndIntegration
907	Amortisation in respect of internally developed computer software and operating expenses	Concept (Monetary)	For Period	Debit	ifrs-full:AmortisationExpense
908	Internal cost	Concept (Monetary)	For Period	Debit	ifrs-full:OperatingExpense
909	100870 - Disclosure - Operating Costs - Summary of Compensation of Key Management Personnel (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureOperatingCostsSummaryOfCompensationOfKeyManagementPersonnelDetail
910	Implied Table	Table	*	*	implied:Table
911	Disclosure Of Operating Costs [Abstract]	Abstract			bt:DisclosureOfOperatingCostsAbstract
912	Short-term employee benefits	Concept (Monetary)	For Period	Debit	ifrs-full:KeyManagementPersonnelCompensationShorttermEmployeeBenefits
913	Post employment benefits	Concept (Monetary)	For Period	Debit	ifrs-full:KeyManagementPersonnelCompensationPostemploymentBenefits
914	Share-based payments	Concept (Monetary)	For Period	Debit	ifrs-full:KeyManagementPersonnelCompensationSharebasedPayment
915	Termination benefits	Concept (Monetary)	For Period	Debit	ifrs-full:KeyManagementPersonnelCompensationTerminationBenefits
916	Compensation of key management personnel	Concept (Monetary)	For Period	Debit	ifrs-full:KeyManagementPersonnelCompensation
917	100880 - Disclosure - Employees - Summary of Number of Employees in the Group (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureEmployeesSummaryOfNumberOfEmployeesInGroupDetail
918	Disclosure Of Number And Average Number Of Employees [Table]	Table	*	*	bt:DisclosureOfNumberAndAverageNumberOfEmployeesTable
919	Geographical areas [axis]	Axis			ifrs-full:GeographicalAreasAxis
920	Geographical areas [member]	Member			ifrs-full:GeographicalAreasMember
921	UK [Member]	Member			country:GB
922	Non-United Kingdom [Member]	Member			bt:NonUnitedKingdomMember
923	Segments [axis]	Axis			ifrs-full:SegmentsAxis
924	Segments [member]	Member			ifrs-full:SegmentsMember
925	BT Consumer [Member]	Member			bt:BTConsumerMember
926	EE [Member]	Member			bt:EELimitedMember
927	Business and Public Sector [Member]	Member			bt:BusinessAndPublicSectorMember
928	Global Services [Member]	Member			bt:GlobalServicesMember
929	Wholesale and Ventures [Member]	Member			bt:WholesaleAndVenturesMember
930	Openreach [Member]	Member			bt:OpenreachMember
931	Other [Member]	Member			ifrs-full:AllOtherSegmentsMember
932	Disclosure Of Number And Average Number Of Employees [Line Items]	LineItems			bt:DisclosureOfNumberAndAverageNumberOfEmployeesLineItems
933	Year end	Concept (Decimal)	As Of		ifrs-full:NumberOfEmployees
934	Average	Concept (Decimal)	For Period		ifrs-full:AverageNumberOfEmployees
935	100890 - Disclosure - Audit, Audit Related and Other Non-Audit Services - Summary of Fees Paid or Payable to Company's Auditors (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureAuditAuditRelatedAndOtherNonAuditServicesSummaryOfFeesPaidOrPayableToCompanySAuditorsDetail
936	Disclosure Of Transactions Between Related Parties [Table]	Table	*	*	ifrs-full:DisclosureOfTransactionsBetweenRelatedPartiesTable
937	Products and services [axis]	Axis			ifrs-full:ProductsAndServicesAxis
938	Products and services [member]	Member			ifrs-full:ProductsAndServicesMember
939	Audit Related Assurance Services	Member			bt:AuditRelatedAssuranceServicesMember
940	Tax Compliance Services	Member			bt:TaxComplianceServicesMember
941	Tax Advisory Services	Member			bt:TaxAdvisoryServicesMember
942	Other Assurance Services	Member			bt:OtherAssuranceServicesMember
943	All Other Non Audit Services	Member			bt:AllOtherNonAuditServicesMember
944	Categories of related parties [axis]	Axis			ifrs-full:CategoriesOfRelatedPartiesAxis
945	Entity's total for related parties [member]	Member			ifrs-full:EntitysTotalForRelatedPartiesMember
946	BT Group plc [Member]	Member			ifrs-full:ParentMember
947	Subsidiaries	Member			ifrs-full:SubsidiariesMember
948	Disclosure Of Transactions Between Related Parties [Line Items]	LineItems			ifrs-full:DisclosureOfTransactionsBetweenRelatedPartiesLineItems
949	Audit services fees	Concept (Monetary)	For Period	Debit	ifrs-full:AuditorsRemunerationForAuditServices
950	Other non-audit services fees	Concept (Monetary)	For Period	Debit	ifrs-full:AuditorsRemunerationForOtherServices
951	Other non-audit tax services fees	Concept (Monetary)	For Period	Debit	ifrs-full:AuditorsRemunerationForTaxServices
952	Total services	Concept (Monetary)	For Period	Debit	ifrs-full:AuditorsRemuneration
953	100900 - Disclosure - Audit, Audit Related and Other Non-Audit Services - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureAuditAuditRelatedAndOtherNonAuditServicesAdditionalInformationDetail
954	Auditor Remuneration [Table]	Table	*	*	bt:AuditorRemunerationTable
955	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
956	Defined benefit plans [member]	Member			ifrs-full:DefinedBenefitPlansMember
957	BT Pension Scheme [Member]	Member			bt:BtPensionSchemeMember
958	Auditor Remuneration [Line Items]	LineItems			bt:AuditorRemunerationLineItems
959	Total fees paid to auditors	Concept (Monetary)	For Period	Debit	bt:AuditFeesForPensionServices
960	100910 - Disclosure - Audit, Audit Related and Other Non-Audit Services - Summary of Total Fees Received by Company's Auditors from the BT Pension Scheme (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureAuditAuditRelatedAndOtherNonAuditServicesSummaryOfTotalFeesReceivedByCompanySAuditorsFromBTPensionSchemeDetail
961	Auditor Remuneration [Table]	Table	*	*	bt:AuditorRemunerationTable
962	Products and services [axis]	Axis			ifrs-full:ProductsAndServicesAxis
963	Products and services [member]	Member			ifrs-full:ProductsAndServicesMember
964	Audit Related Assurance Services	Member			bt:AuditRelatedAssuranceServicesMember
965	Tax Compliance Services	Member			bt:TaxComplianceServicesMember
966	Tax Advisory Services	Member			bt:TaxAdvisoryServicesMember
967	All Other Non Audit Services	Member			bt:AllOtherNonAuditServicesMember
968	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
969	Defined benefit plans [member]	Member			ifrs-full:DefinedBenefitPlansMember
970	BT Pension Scheme [Member]	Member			bt:BtPensionSchemeMember
971	Categories of related parties [axis]	Axis			ifrs-full:CategoriesOfRelatedPartiesAxis
972	Entity's total for related parties [member]	Member			ifrs-full:EntitysTotalForRelatedPartiesMember
973	Audit Of Financial Statements Of Associates [Member]	Member			bt:AuditOfFinancialStatementsOfAssociatesMember
974	Auditor Remuneration [Line Items]	LineItems			bt:AuditorRemunerationLineItems
975	Audit services fees	Concept (Monetary)	For Period	Debit	ifrs-full:AuditorsRemunerationForAuditServices
976	Audit-related assurance services	Concept (Monetary)	For Period	Debit	ifrs-full:AuditorsRemunerationForOtherServices
977	Other non-audit tax services fees	Concept (Monetary)	For Period	Debit	ifrs-full:AuditorsRemunerationForTaxServices
978	Total services	Concept (Monetary)	For Period	Debit	ifrs-full:AuditorsRemuneration
979	100920 - Disclosure - Specific Items - Summary of Specific Items (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureSpecificItemsSummaryOfSpecificItemsDetail
980	Disclosure Of Specific Items [Table]	Table	*	*	bt:DisclosureOfSpecificItemsTable
981	Retrospective application and retrospective restatement [axis]	Axis			ifrs-full:RetrospectiveApplicationAndRetrospectiveRestatementAxis
982	Currently stated [member]	Member			ifrs-full:RestatedMember
983	Specific Items [Member]	Member			bt:SpecificItemsMember
984	Disclosure Of Specific Items [Line Items]	LineItems			bt:DisclosureOfSpecificItemsLineItems
985	Revenue	Abstract			bt:RevenueFromSpecificItemsAbstract
986	Italian business investigation	Concept (Monetary)	For Period	Credit	bt:BusinessInvestigationRevenue
987	Regulatory matters	Concept (Monetary)	For Period	Credit	bt:RegulatoryMattersRevenue
988	EE fair value adjustment	Concept (Monetary)	For Period	Credit	bt:FairValueAdjustmentRevenue
989	Total revenue	Concept (Monetary)	For Period	Credit	ifrs-full:Revenue
990	Operating costs	Abstract			bt:OperatingCostsFromSpecificItemsAbstract
991	EE acquisition warranty claims	Concept (Monetary)	For Period	Debit	bt:AcquisitionWarrantyClaims
992	Restructuring charges	Concept (Monetary)	For Period	Debit	ifrs-full:ExpenseOfRestructuringActivities
993	EE acquisition and integration costs	Concept (Monetary)	For Period	Debit	bt:AcquisitionAndIntegrationCosts
994	Property rationalisation costs	Concept (Monetary)	For Period	Debit	bt:PropertyRationalisationCosts
995	Regulatory matters	Concept (Monetary)	For Period	Debit	bt:RegulatoryMattersOperatingCosts
996	Italian business investigation	Concept (Monetary)	For Period	Debit	bt:BusinessInvestigationOperatingCosts
997	Out of period irrecoverable VAT	Concept (Monetary)	For Period	Debit	bt:OutOfPeriodIrrecoverableVat
998	Profit on disposal of businesses	Concept (Monetary)	For Period	Credit	ifrs-full:GainsOnDisposalsOfInvestments
999	Total operating costs	Concept (Monetary)	For Period	Debit	ifrs-full:OperatingExpense
1000	Operating loss	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLossFromOperatingActivities
1001	Net finance expense	Abstract			bt:NetFinanceExpenseFromSpecificItemsAbstract
1002	Interest expense on retirement benefit obligation	Concept (Monetary)	For Period	Credit	ifrs-full:InterestExpenseIncomeNetDefinedBenefitLiabilityAsset
1003	EE related finance cost	Concept (Monetary)	For Period	Debit	bt:AcquisitionRelatedFinanceCost
1004	Interest on out of period irrecoverable VAT	Concept (Monetary)	For Period	Credit	bt:InterestOnOutOfPeriodIrrecoverableVat
1005	Total net finance expense	Concept (Monetary)	For Period	Credit	ifrs-full:FinanceIncomeCost
1006	Net specific items charge before tax	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLossBeforeTax
1007	Taxation	Abstract			bt:TaxationFromSpecificItemsAbstract
1008	Tax credit on specific items above	Concept (Monetary)	For Period	Credit	bt:TaxCreditOnSpecificItems
1009	Tax credit on re-measurement of deferred tax	Concept (Monetary)	For Period	Credit	bt:TaxCreditOnRemeasurementOfDeferredTax
1010	Total taxation	Concept (Monetary)	For Period	Debit	ifrs-full:IncomeTaxExpenseContinuingOperations
1011	Net specific items charge after tax	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLoss
1012	100930 - Disclosure - Specific Items - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureSpecificItemsAdditionalInformationDetail
1013	Disclosure Of Specific Items [Table]	Table	*	*	bt:DisclosureOfSpecificItemsTable
1014	Retrospective application and retrospective restatement [axis]	Axis			ifrs-full:RetrospectiveApplicationAndRetrospectiveRestatementAxis
1015	Currently stated [member]	Member			ifrs-full:RestatedMember
1016	Specific Items [Member]	Member			bt:SpecificItemsMember
1017	Disclosure Of Specific Items [Line Items]	LineItems			bt:DisclosureOfSpecificItemsLineItems
1018	EE acquisition warranty claim	Concept (Monetary)	For Period	Debit	bt:AcquisitionWarrantyClaims
1019	Restructuring charges	Concept (Monetary)	For Period	Debit	ifrs-full:ExpenseOfRestructuringActivities
1020	Expected restructuring charges	Concept (Monetary)	For Period	Debit	bt:ExpectedRestructuringCharges
1021	EE acquisition and integration costs	Concept (Monetary)	For Period	Debit	bt:AcquisitionAndIntegrationCosts
1022	Amortisation charge relating to write-off of IT assets	Concept (Monetary)	For Period	Debit	bt:AmortisationChargeRelatingToWriteoffOfAssets
1023	Property rationalisation costs	Concept (Monetary)	For Period	Debit	bt:PropertyRationalisationCosts
1024	Regulatory matters effect	Concept (Monetary)	For Period	Credit	bt:RegulatoryMattersProfitLoss
1025	Regulatory matters revenue	Concept (Monetary)	For Period	Credit	bt:RegulatoryMattersRevenue
1026	Specific operating costs	Concept (Monetary)	For Period	Debit	bt:SpecificOperatingCosts
1027	Deemed consent charge	Concept (Monetary)	For Period	Debit	bt:DeemedConsentCharge
1028	Regulatory matters effect	Concept (Monetary)	For Period	Debit	bt:DeemedConsentChargeFineAmount
1029	Deemed consent charge, net	Concept (Monetary)	For Period	Debit	bt:DeemedConsentChargeNet
1030	Ladder pricing agreements revenue recognised	Concept (Monetary)	For Period	Credit	bt:RevenueRecognisedInLadderPricingAgreements
1031	Ladder pricing agreements costs recognised	Concept (Monetary)	For Period	Credit	bt:CostsRecognisedInLadderPricingAgreements
1032	Other regulatory matters	Concept (Monetary)	For Period	Debit	bt:OtherRegulatoryMatters
1033	Provision for other regulatory matters	Concept (Monetary)	For Period	Credit	bt:ProvisionForOtherRegulatoryMatters
1034	Trade and other payables	Concept (Monetary)	As Of	Credit	ifrs-full:TradeAndOtherPayables
1035	Italian business investigation	Concept (Monetary)	For Period	Debit	bt:BusinessInvestigationOperatingCosts
1036	Charge recognised related to investigation of specific items	Concept (Monetary)	For Period	Debit	bt:ChargeRecognisedRelatedToInvestigationOfSpecificItems
1037	Fees incurred related to investigation of specific items	Concept (Monetary)	For Period	Debit	bt:FeesIncurredRelatedToInvestigationOfSpecificItems
1038	Profit on disposal of businesses	Concept (Monetary)	For Period	Credit	ifrs-full:GainsOnDisposalsOfInvestments
1039	EE fair value adjustment	Concept (Monetary)	For Period	Credit	bt:FairValueAdjustmentRevenue
1040	Out of period irrecoverable VAT	Concept (Monetary)	For Period	Debit	bt:OutOfPeriodIrrecoverableVat
1041	Interest on out of period irrecoverable VAT	Concept (Monetary)	For Period	Credit	bt:InterestOnOutOfPeriodIrrecoverableVat
1042	100940 - Disclosure - Taxation - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTaxationAdditionalInformationDetail
1043	Disclosure Of Income Tax Expense [Table]	Table	*	*	bt:DisclosureOfIncomeTaxExpenseTable
1044	Categories of related parties [axis]	Axis			ifrs-full:CategoriesOfRelatedPartiesAxis
1045	Entity's total for related parties [member]	Member			ifrs-full:EntitysTotalForRelatedPartiesMember
1046	EE [Member]	Member			bt:EELimitedMember
1047	Non-adjusting events after reporting period [axis]	Axis			ifrs-full:NonadjustingEventsAfterReportingPeriodAxis
1048	Non-adjusting events after reporting period [member]	Member			ifrs-full:NonadjustingEventsMember
1049	Events After Reporting Period [Member]	Member			bt:EventsAfterReportingPeriodMember
1050	Geographical areas [axis]	Axis			ifrs-full:GeographicalAreasAxis
1051	Geographical areas [member]	Member			ifrs-full:GeographicalAreasMember
1052	UK [Member]	Member			country:GB
1053	Non-United Kingdom [Member]	Member			bt:NonUnitedKingdomMember
1054	European Union [Member]	Member			bt:EuropeanUnionOneMember
1055	Range [axis]	Axis			ifrs-full:RangeAxis
1056	Ranges [member]	Member			ifrs-full:RangesMember
1057	Top of Range [Member]	Member			ifrs-full:TopOfRangeMember
1058	Bottom of Range [Member]	Member			ifrs-full:BottomOfRangeMember
1059	Disclosure Of Income Tax Expense [Line Items]	LineItems			bt:DisclosureOfIncomeTaxExpenseLineItems
1060	Approximate percentage of provision under tax authority examination	Concept (Percent)	As Of		bt:ApproximatePercentageOfProvisionUnderTaxAuthorityExamination
1061	Provision for tax uncertainties included in current tax liabilities	Concept (Monetary)	As Of	Credit	bt:ProvisionForTaxUncertaintiesIncludedInCurrentTaxLiabilities
1062	Deferred tax asset on historical tax losses	Concept (Monetary)	As Of	Debit	bt:DeferredTaxAssetsTaxLossesCarriedForward
1063	Additional amount required for tax settlement	Concept (Monetary)	As Of	Credit	bt:AdditionalAmountRequiredForTaxSettlement
1064	Current tax expense	Concept (Monetary)	For Period	Debit	ifrs-full:CurrentTaxExpenseIncome
1065	Corporation tax	Concept (Percent)	For Period		ifrs-full:TaxRateEffectFromChangeInTaxRate
1066	Deferred tax credit specific item recognised	Concept (Monetary)	For Period	Credit	bt:TaxCreditOnRemeasurementOfDeferredTax
1067	Unused tax losses for which no deferred tax asset recognised	Concept (Monetary)	As Of		ifrs-full:UnusedTaxLossesForWhichNoDeferredTaxAssetRecognised
1068	Deferred tax liability	Concept (Monetary)	As Of	Credit	bt:DeferredTaxOnUnremittedEarnings
1069	Retained profit (loss)	Concept (Monetary)	As Of	Credit	ifrs-full:RetainedEarnings
1070	Additional tax that would be incurred if earnings were to be repatriated	Concept (Monetary)	For Period	Debit	bt:AdditionalTaxThatWouldBeIncurredIfEarningsWereToBeRepatriated
1071	Additional income tax	Concept (Monetary)	For Period		bt:AdditionalIncomeTax
1072	100950 - Disclosure - Taxation - Analysis of Our Taxation Expenses (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTaxationAnalysisOfOurTaxationExpensesDetail
1073	Disclosure Of Income Tax Expense [Table]	Table	*	*	bt:DisclosureOfIncomeTaxExpenseTable
1074	Geographical areas [axis]	Axis			ifrs-full:GeographicalAreasAxis
1075	Geographical areas [member]	Member			ifrs-full:GeographicalAreasMember
1076	UK [Member]	Member			country:GB
1077	Non-United Kingdom [Member]	Member			bt:NonUnitedKingdomMember
1078	Disclosure Of Income Tax Expense [Line Items]	LineItems			bt:DisclosureOfIncomeTaxExpenseLineItems
1079	Current	Concept (Monetary)	For Period	Debit	ifrs-full:CurrentTaxExpenseIncome
1080	Origination and reversal of temporary differences	Concept (Monetary)	For Period	Debit	ifrs-full:DeferredTaxExpenseIncomeRelatingToOriginationAndReversalOfTemporaryDifferences
1081	Adjustments in respect of earlier years	Concept (Monetary)	For Period	Debit	ifrs-full:AdjustmentsForCurrentTaxOfPriorPeriod
1082	Total current tax expense	Concept (Monetary)	For Period	Debit	ifrs-full:CurrentTaxExpenseIncomeAndAdjustmentsForCurrentTaxOfPriorPeriods
1083	Impact of change in UK corporation tax rate to 17% (2016/17: 17%, 2015/16: 19%)	Concept (Monetary)	For Period	Debit	ifrs-full:DeferredTaxExpenseIncomeRelatingToTaxRateChangesOrImpositionOfNewTaxes
1084	Remeasurement of temporary differences	Concept (Monetary)	For Period	Debit	ifrs-full:AdjustmentsForDeferredTaxOfPriorPeriods
1085	Total deferred taxation credit	Concept (Monetary)	For Period	Debit	bt:TotalDeferredTaxationCredit
1086	Total taxation expense	Concept (Monetary)	For Period	Debit	ifrs-full:IncomeTaxExpenseContinuingOperations
1087	100960 - Disclosure - Taxation - Analysis of Our Taxation Expenses (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTaxationAnalysisOfOurTaxationExpensesParentheticalDetail
1088	Disclosure Of Income Tax Expense [Table]	Table	*	*	bt:DisclosureOfIncomeTaxExpenseTable
1089	Geographical areas [axis]	Axis			ifrs-full:GeographicalAreasAxis
1090	Geographical areas [member]	Member			ifrs-full:GeographicalAreasMember
1091	UK [Member]	Member			country:GB
1092	Disclosure Of Income Tax Expense [Line Items]	LineItems			bt:DisclosureOfIncomeTaxExpenseLineItems
1093	Corporation tax rate	Concept (Percent)	For Period		ifrs-full:ApplicableTaxRate
1094	Change in corporation tax rate	Concept (Percent)	For Period		ifrs-full:TaxRateEffectFromChangeInTaxRate
1095	100970 - Disclosure - Taxation - Summary of Factors Affecting Our Taxation Expense for the Year (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTaxationSummaryOfFactorsAffectingOurTaxationExpenseForYearDetail
1096	Implied Table	Table	*	*	implied:Table
1097	Disclosure Of Factors Affecting Our Taxation Expense For The Year [Abstract]	Abstract			bt:DisclosureOfFactorsAffectingOurTaxationExpenseForTheYearAbstract
1098	Profit before taxation	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLossBeforeTax
1099	Expected taxation expense at UK rate of 19% (2016/17: 20%, 2015/16: 20%)	Concept (Monetary)	For Period	Debit	bt:ExpectedTaxExpenseAtUKStatutoryRate
1100	(Higher) lower taxes on non-UK profits	Concept (Monetary)	For Period	Debit	bt:EffectOfChangeInTaxOnNonUnitedKingdomProfits
1101	Net permanent differences between tax and accounting	Concept (Monetary)	For Period	Debit	bt:EffectOfNetPermanentDifferencesBetweenTaxAndAccounting
1102	Adjustments in respect of earlier years	Concept (Monetary)	For Period	Debit	bt:EffectOfAdjustmentsInRespectOfEarlierYears
1103	Prior year non-UK losses used against current year profits	Concept (Monetary)	For Period	Debit	bt:EffectOfNonUnitedKingdomLossesUsedAgainstCurrentYearProfits
1104	Non-UK losses not recognised	Concept (Monetary)	For Period	Debit	bt:EffectOfNonUnitedKingdomLossesNotRecognised
1105	Other deferred tax assets not recognised	Concept (Monetary)	For Period	Debit	bt:EffectOfOtherDeferredTaxAssetsNotRecognised
1106	Lower taxes on profit on disposal of business	Concept (Monetary)	For Period	Debit	bt:EffectOfLowerTaxesOnProfitOnDisposalOfBusiness
1107	Re-measurement of deferred tax balances	Concept (Monetary)	For Period	Debit	bt:EffectOfRemeasurementOfDeferredTaxBalances
1108	Other non-recurring items	Concept (Monetary)	For Period	Debit	bt:EffectOfOtherNonrecurringItems
1109	Total taxation expense	Concept (Monetary)	For Period	Debit	ifrs-full:IncomeTaxExpenseContinuingOperations
1110	Exclude specific items (note 8)	Concept (Monetary)	For Period	Debit	bt:TaxExpenseIncomeContinuingOperationsExcludeSpecificItems
1111	Total taxation expense before specific items	Concept (Monetary)	For Period	Debit	bt:IncomeTaxExpenseContinuingOperationsBeforeSpecificItems
1112	100980 - Disclosure - Taxation - Summary of Factors Affecting Our Taxation Expense for the Year (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTaxationSummaryOfFactorsAffectingOurTaxationExpenseForYearParentheticalDetail
1113	Implied Table	Table	*	*	implied:Table
1114	Disclosure Of Factors Affecting Our Taxation Expense For The Year [Abstract]	Abstract			bt:DisclosureOfFactorsAffectingOurTaxationExpenseForTheYearAbstract
1115	Tax rate	Concept (Percent)	For Period		ifrs-full:ApplicableTaxRate
1116	100990 - Disclosure - Taxation - Summary of Tax Components of Other Comprehensive Income (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTaxationSummaryOfTaxComponentsOfOtherComprehensiveIncomeDetail
1117	Implied Table	Table	*	*	implied:Table
1118	Income Tax Relating To Components Of Other Comprehensive Income [Abstract]	Abstract			ifrs-full:IncomeTaxRelatingToComponentsOfOtherComprehensiveIncomeAbstract
1119	Tax on pension remeasurements	Concept (Monetary)	For Period	Debit	ifrs-full:IncomeTaxRelatingToRemeasurementsOfDefinedBenefitPlansOfOtherComprehensiveIncome
1120	Exchange differences on translation of foreign operations	Concept (Monetary)	For Period	Debit	ifrs-full:IncomeTaxRelatingToExchangeDifferencesOnTranslationOfOtherComprehensiveIncome
1121	Fair value movements on cash flow hedges - net fair value gains or losses	Concept (Monetary)	For Period	Debit	bt:IncomeTaxRelatingToComponentsOfOtherComprehensiveIncomeFairValueMovementsOnCashFlowHedgesGainOrLosses
1122	Fair value movements on cash flow hedges - recognised in income and expense	Concept (Monetary)	For Period	Debit	bt:IncomeTaxRelatingToComponentsOfOtherComprehensiveIncomeFairValueMovementsOnCashFlowHedgesRecognisedInIncomeAndExpense
1123	Income tax relating to components of other comprehensive income	Concept (Monetary)	For Period	Debit	bt:IncomeTaxRelatingToComponentsOfIncomeStatement
1124	Current tax credit	Concept (Monetary)	For Period	Debit	bt:CurrentIncomeAndSocialContributionTaxCredits
1125	Deferred tax (expense) credit	Concept (Monetary)	For Period	Debit	ifrs-full:DeferredTaxExpenseIncome
1126	Tax on other comprehensive income (loss)	Concept (Monetary)	For Period		ifrs-full:IncomeTaxRelatingToComponentsOfOtherComprehensiveIncome
1127	101000 - Disclosure - Taxation - Summary of Tax Components of Other Comprehensive Income (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTaxationSummaryOfTaxComponentsOfOtherComprehensiveIncomeParentheticalDetail
1128	Implied Table	Table	*	*	implied:Table
1129	Income Tax Relating To Components Of Other Comprehensive Income [Abstract]	Abstract			ifrs-full:IncomeTaxRelatingToComponentsOfOtherComprehensiveIncomeAbstract
1130	Current tax credit includes cash contributions made to reduce retirement benefit obligations	Concept (Monetary)	For Period	Credit	bt:CurrentTaxCreditIncludesCashContributionsMadeToReduceRetirementBenefitObligations
1131	101010 - Disclosure - Taxation - Summary of Tax (Expense) Credit Recognised Directly in Equity (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTaxationSummaryOfTaxExpenseCreditRecognisedDirectlyInEquityDetail
1132	Implied Table	Table	*	*	implied:Table
1133	Disclosure Of Tax Expense Credit Recognised Directly In Equity [Abstract]	Abstract			bt:DisclosureOfTaxExpenseCreditRecognisedDirectlyInEquityAbstract
1134	Tax (expense) credit relating to share-based payments	Concept (Monetary)	For Period	Debit	bt:TaxEffectRelatedToShareBasedPayments
1135	101020 - Disclosure - Taxation - Summary of Deferred Taxation (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTaxationSummaryOfDeferredTaxationDetail
1136	Disclosure Of Temporary Difference Unused Tax Losses And Unused Tax Credits [Table]	Table	*	*	ifrs-full:DisclosureOfTemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsTable
1137	Temporary difference, unused tax losses and unused tax credits [axis]	Axis			ifrs-full:TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsAxis
1138	Temporary difference, unused tax losses and unused tax credits [member]	Member			ifrs-full:TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsMember
1139	Fixed Asset Temporary Differences [Member]	Member			bt:FixedAssetTemporaryDifferencesMember
1140	Retirement Benefit Obligations [Member]	Member			bt:RetirementBenefitObligationsMember
1141	Share-Based Payments [Member]	Member			bt:SharebasedPaymentsMember
1142	Tax Losses [Member]	Member			bt:TaxLossesMember
1143	Other [Member]	Member			bt:OtherMember
1144	Jurisdictional Offset [Member]	Member			bt:JurisdictionalOffsetMember
1145	Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]	LineItems			ifrs-full:DisclosureOfTemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsLineItems
1146	Beginning balance	Concept (Monetary)	As Of	Credit	ifrs-full:DeferredTaxLiabilityAsset
1147	Reclassification between categories	Concept (Monetary)	For Period	Credit	bt:DeferredTaxExpenseIncomeReclassificationBetweenCategories
1148	(Credit) expense recognised in the income statement	Concept (Monetary)	For Period		ifrs-full:DeferredTaxExpenseIncomeRecognisedInProfitOrLoss
1149	(Credit) expense recognised in other comprehensive income	Concept (Monetary)	For Period	Debit	ifrs-full:OtherComponentsOfDeferredTaxExpenseIncome
1150	Expense recognised in equity	Concept (Monetary)	For Period		ifrs-full:DeferredTaxRelatingToItemsChargedOrCreditedDirectlyToEquity
1151	Exchange differences	Concept (Monetary)	For Period	Credit	ifrs-full:IncreaseDecreaseThroughNetExchangeDifferencesDeferredTaxLiabilityAsset
1152	Transfer to current tax	Concept (Monetary)	For Period	Credit	bt:IncreaseDecreaseThroughTransferDeferredTaxLiabilityAsset
1153	Ending balance	Concept (Monetary)	As Of	Credit	ifrs-full:DeferredTaxLiabilityAsset
1154	Deferred tax asset	Concept (Monetary)	As Of	Debit	ifrs-full:DeferredTaxAssets
1155	Deferred tax liability	Concept (Monetary)	As Of	Credit	ifrs-full:DeferredTaxLiabilities
1156	101030 - Disclosure - Taxation - Summary of Deferred Taxation (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTaxationSummaryOfDeferredTaxationParentheticalDetail
1157	Disclosure Of Temporary Difference Unused Tax Losses And Unused Tax Credits [Table]	Table	*	*	ifrs-full:DisclosureOfTemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsTable
1158	Temporary difference, unused tax losses and unused tax credits [axis]	Axis			ifrs-full:TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsAxis
1159	Temporary difference, unused tax losses and unused tax credits [member]	Member			ifrs-full:TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsMember
1160	Defined Contribution Plan [Member]	Member			bt:DefinedContributionPlanMember
1161	Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]	LineItems			ifrs-full:DisclosureOfTemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsLineItems
1162	Net deferred tax	Concept (Monetary)	As Of	Credit	ifrs-full:DeferredTaxLiabilityAsset
1163	101040 - Disclosure - Taxation - Summary of Restricted Loss (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTaxationSummaryOfRestrictedLossDetail
1164	Disclosure Of Unrecognised Tax Losses And Other Temporary Differences [Table]	Table	*	*	bt:DisclosureOfUnrecognisedTaxLossesAndOtherTemporaryDifferencesTable
1165	Geographical areas [axis]	Axis			ifrs-full:GeographicalAreasAxis
1166	Geographical areas [member]	Member			ifrs-full:GeographicalAreasMember
1167	Europe [Member]	Member			bt:EuropeOneMember
1168	Americas [Member]	Member			country:US
1169	Other Countries [Member]	Member			bt:OtherCountriesMember
1170	Range [axis]	Axis			ifrs-full:RangeAxis
1171	Ranges [member]	Member			ifrs-full:RangesMember
1172	Bottom of Range [Member]	Member			ifrs-full:BottomOfRangeMember
1173	Top of Range [Member]	Member			ifrs-full:TopOfRangeMember
1174	Disclosure Of Unrecognised Tax Losses And Other Temporary Differences [Line Items]	LineItems			bt:DisclosureOfUnrecognisedTaxLossesAndOtherTemporaryDifferencesLineItems
1175	Deferred tax assets restricted losses expiry date	Concept (Year)	For Period		bt:DeferredTaxAssetsRestrictedlossesExpiry
1176	Total restricted losses	Concept (Monetary)	As Of	Debit	bt:DeferredTaxAssetsRestrictedlosses
1177	Unrestricted operating losses	Concept (Monetary)	As Of	Debit	bt:DeferredTaxAssetUnrestrictedOperatingLosses
1178	Other temporary differences	Concept (Monetary)	As Of	Debit	bt:DeferredTaxAssetsOtherTemporaryDifferences
1179	Total	Concept (Monetary)	As Of	Debit	bt:DeferredTaxAssetsUnrestrictedOperatingLossesAndOtherTemporaryDifferences
1180	101050 - Disclosure - Earnings Per Share - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureEarningsPerShareAdditionalInformationDetail
1181	Implied Table	Table	*	*	implied:Table
1182	Profit Loss Attributable To Ordinary Equity Holders Of Parent Entity [Abstract]	Abstract			ifrs-full:ProfitLossAttributableToOrdinaryEquityHoldersOfParentEntityAbstract
1183	Diluted earnings per share, share options	Concept (Shares)	For Period		bt:SharesExcludedFromCalculationOfDilutedNumberOfShares
1184	Profit for the year	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLoss
1185	Profit (loss), attributable to non-controlling interests	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLossAttributableToNoncontrollingInterests
1186	101060 - Disclosure - Earnings Per Share - Schedule of Calculation of the Total Diluted Number of Shares (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureEarningsPerShareScheduleOfCalculationOfTotalDilutedNumberOfSharesDetail
1187	Implied Table	Table	*	*	implied:Table
1188	Profit Loss Attributable To Ordinary Equity Holders Of Parent Entity [Abstract]	Abstract			ifrs-full:ProfitLossAttributableToOrdinaryEquityHoldersOfParentEntityAbstract
1189	Basic weighted average number of shares (millions)	Concept (Shares)	For Period		ifrs-full:WeightedAverageShares
1190	Dilutive shares from share options (millions)	Concept (Shares)	For Period		ifrs-full:DilutiveEffectOfShareOptionsOnNumberOfOrdinaryShares
1191	Dilutive shares from executive share awards (millions)	Concept (Shares)	For Period		bt:DilutiveSharesFromExecutiveShareAwards
1192	Diluted weighted average number of shares (millions)	Concept (Shares)	For Period		ifrs-full:AdjustedWeightedAverageShares
1193	Basic	Concept (Share)	For Period		ifrs-full:BasicEarningsLossPerShare
1194	Diluted	Concept (Share)	For Period		ifrs-full:DilutedEarningsLossPerShare
1195	101070 - Disclosure - Dividends - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureDividendsAdditionalInformationDetail
1196	Implied Table	Table	*	*	implied:Table
1197	Disclosure Of Dividends Paid And Proposed [Abstract]	Abstract			bt:DisclosureOfDividendsPaidAndProposedAbstract
1198	Final dividend payable per share	Concept (Share)	For Period		bt:FinalDividendProposedAsDistributionsToOwnersPerShare
1199	Final dividend payable date	Concept (Date)	For Period		bt:DividendsPaidDate
1200	Proposed dividend payable per share	Concept (Share)	For Period		bt:DividendsProposedAsDistributionsToOwnersPerShare
1201	Proposed dividend payable amount	Concept (Monetary)	For Period		ifrs-full:DividendsProposedOrDeclaredBeforeFinancialStatementsAuthorisedForIssueButNotRecognisedAsDistributionToOwners
1202	Final dividend payable amount	Concept (Monetary)	As Of	Credit	ifrs-full:DividendPayables
1203	Proposed dividend payable date	Concept (Date)	For Period		bt:DividendPayableDate
1204	Total dividend paid amount	Concept (Monetary)	For Period	Debit	ifrs-full:DividendsRecognisedAsDistributionsToOwnersOfParent
1205	101080 - Disclosure - Dividends - Summary of Final and Interim Dividend Per Share and Amount (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureDividendsSummaryOfFinalAndInterimDividendPerShareAndAmountDetail
1206	Implied Table	Table	*	*	implied:Table
1207	Disclosure Of Dividends [Abstract]	Abstract			bt:DisclosureOfDividendsAbstract
1208	Final dividend in respect of the prior year, per share	Concept (Share)	For Period		bt:DividendsPerShareAsDistributionsToOwnersOfParentRelatingToPriorYears
1209	Interim dividend in respect of the current year, per share	Concept (Share)	For Period		bt:DividendsPerShareAsDistributionsToOwnersOfParentRelatingToCurrentYear
1210	Total dividend per share	Concept (Share)	For Period		ifrs-full:DividendsRecognisedAsDistributionsToOwnersPerShare
1211	Final dividend in respect of the prior year, amount	Concept (Monetary)	For Period	Debit	ifrs-full:DividendsRecognisedAsDistributionsToOwnersOfParentRelatingToPriorYears
1212	Interim dividend in respect of the current year, amount	Concept (Monetary)	For Period	Debit	ifrs-full:DividendsRecognisedAsDistributionsToOwnersOfParentRelatingToCurrentYear
1213	Total dividend paid amount	Concept (Monetary)	For Period	Debit	ifrs-full:DividendsRecognisedAsDistributionsToOwnersOfParent
1214	101090 - Disclosure - Intangible Assets - Summary of Intangible Assets (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureIntangibleAssetsSummaryOfIntangibleAssetsDetail
1215	Disclosure Of Reconciliation Of Changes In Intangible Assets And Goodwill [Table]	Table	*	*	ifrs-full:DisclosureOfReconciliationOfChangesInIntangibleAssetsAndGoodwillTable
1216	Classes of intangible assets and goodwill [axis]	Axis			ifrs-full:ClassesOfIntangibleAssetsAndGoodwillAxis
1217	Intangible assets and goodwill [member]	Member			ifrs-full:IntangibleAssetsAndGoodwillMember
1218	Goodwill [Member]	Member			ifrs-full:GoodwillMember
1219	Customer Relationships and Brands [Member]	Member			ifrs-full:CustomerrelatedIntangibleAssetsMember
1220	Telecoms Licences and Other [Member]	Member			bt:TelecomsLicencesAndOtherMember
1221	Internally Developed Software [Member]	Member			bt:InternallyDevelopedSoftwareMember
1222	Purchased Software [Member]	Member			bt:PurchasedSoftwareMember
1223	Carrying amount, accumulated depreciation, amortisation and impairment and gross carrying amount [axis]	Axis			ifrs-full:CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis
1224	Carrying amount [member]	Member			ifrs-full:CarryingAmountMember
1225	Gross Carrying Amount [Member]	Member			ifrs-full:GrossCarryingAmountMember
1226	Accumulated Depreciation and Amortisation [Member]	Member			ifrs-full:AccumulatedDepreciationAndAmortisationMember
1227	Disclosure of reconciliation of changes in intangible assets and goodwill [Line Items]	LineItems			ifrs-full:DisclosureOfReconciliationOfChangesInIntangibleAssetsAndGoodwillLineItems
1228	Beginning balance	Concept (Monetary)	As Of	Debit	ifrs-full:IntangibleAssetsAndGoodwill
1229	Acquisitions	Concept (Monetary)	For Period	Debit	ifrs-full:AcquisitionsThroughBusinessCombinationsIntangibleAssetsAndGoodwill
1230	Charge for the year	Concept (Monetary)	For Period	Debit	bt:AmortisationIntangibleAssetsOtherThanGoodwillAfterAdjustingSpecificItems
1231	Additions	Concept (Monetary)	For Period	Debit	ifrs-full:AdditionsOtherThanThroughBusinessCombinationsIntangibleAssetsOtherThanGoodwill
1232	Disposals and adjustments	Concept (Monetary)	For Period	Credit	ifrs-full:DisposalsAndRetirementsIntangibleAssetsAndGoodwill
1233	Transfers	Concept (Monetary)	For Period	Debit	ifrs-full:IncreaseDecreaseThroughTransfersIntangibleAssetsAndGoodwill
1234	Exchange differences	Concept (Monetary)	For Period	Debit	ifrs-full:IncreaseDecreaseThroughNetExchangeDifferencesIntangibleAssetsAndGoodwill
1235	Ending balance	Concept (Monetary)	As Of	Debit	ifrs-full:IntangibleAssetsAndGoodwill
1236	101100 - Disclosure - Intangible Assets - Summary of Intangible Assets (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureIntangibleAssetsSummaryOfIntangibleAssetsParentheticalDetail
1237	Disclosure Of Reconciliation Of Changes In Intangible Assets And Goodwill [Table]	Table	*	*	ifrs-full:DisclosureOfReconciliationOfChangesInIntangibleAssetsAndGoodwillTable
1238	Retrospective application and retrospective restatement [axis]	Axis			ifrs-full:RetrospectiveApplicationAndRetrospectiveRestatementAxis
1239	Currently stated [member]	Member			ifrs-full:RestatedMember
1240	Specific Items [Member]	Member			bt:SpecificItemsMember
1241	Disclosure of reconciliation of changes in intangible assets and goodwill [Line Items]	LineItems			ifrs-full:DisclosureOfReconciliationOfChangesInIntangibleAssetsAndGoodwillLineItems
1242	Reduction in cost and accumulated depreciation due to disposal of fixed assets	Concept (Monetary)	For Period	Debit	bt:ReductionInCostAndAccumulatedDepreciationDueToDisposalOfFixedAssets
1243	Amortisation charge related to write off of assets	Concept (Monetary)	For Period	Debit	bt:AmortisationChargeRelatingToWriteoffOfAssets
1244	101110 - Disclosure - Intangible Assets - Summary of Goodwill Allocated to Cash Generating Units (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureIntangibleAssetsSummaryOfGoodwillAllocatedToCashGeneratingUnitsDetail
1245	Disclosure Of Goodwill Allocated To C G Us [Table]	Table	*	*	bt:DisclosureOfGoodwillAllocatedToCGUsTable
1246	Cash-generating units [axis]	Axis			ifrs-full:IndividualAssetsOrCashgeneratingUnitsWithSignificantAmountOfGoodwillOrIntangibleAssetsWithIndefiniteUsefulLivesAxis
1247	Entity's total for cash-generating units [member]	Member			ifrs-full:EntitysTotalForCashgeneratingUnitsMember
1248	Consumer [Member]	Member			bt:ConsumerMember
1249	EE [Member]	Member			bt:EEMember
1250	Business and Public Sector [Member]	Member			bt:BusinessAndPublicSectorMember
1251	Global Services [Member]	Member			bt:GlobalServicesMember
1252	Wholesale and Ventures [Member]	Member			bt:WholesaleAndVenturesMember
1253	Disclosure of goodwill allocated to CGUs [Line Items]	LineItems			bt:DisclosureOfGoodwillAllocatedToCGUsLineItems
1254	Beginning balance	Concept (Monetary)	As Of	Debit	ifrs-full:Goodwill
1255	Re-organisation	Concept (Monetary)	For Period	Debit	ifrs-full:IncreaseDecreaseThroughOtherChangesIntangibleAssetsAndGoodwill
1256	Exchange differences	Concept (Monetary)	For Period	Debit	ifrs-full:IncreaseDecreaseThroughNetExchangeDifferencesIntangibleAssetsAndGoodwill
1257	Acquisitions and disposals	Concept (Monetary)	For Period	Debit	ifrs-full:AcquisitionsThroughBusinessCombinationsIntangibleAssetsAndGoodwill
1258	Acquisitions and disposals	Concept (Monetary)	For Period	Credit	ifrs-full:DisposalsIntangibleAssetsAndGoodwill
1259	Ending balance	Concept (Monetary)	As Of	Debit	ifrs-full:Goodwill
1260	101120 - Disclosure - Intangible Assets - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureIntangibleAssetsAdditionalInformationDetail
1261	Disclosure Of Information For Individual Asset Or Cashgenerating Unit With Significant Amount Of Goodwill Or Intangible Assets With Indefinite Useful Lives [Table]	Table	*	*	ifrs-full:DisclosureOfInformationForIndividualAssetOrCashgeneratingUnitWithSignificantAmountOfGoodwillOrIntangibleAssetsWithIndefiniteUsefulLivesTable
1262	Cash-generating units [axis]	Axis			ifrs-full:IndividualAssetsOrCashgeneratingUnitsWithSignificantAmountOfGoodwillOrIntangibleAssetsWithIndefiniteUsefulLivesAxis
1263	Entity's total for cash-generating units [member]	Member			ifrs-full:EntitysTotalForCashgeneratingUnitsMember
1264	Global Services [Member]	Member			bt:GlobalServicesMember
1265	Business and Public Sector [Member]	Member			bt:BusinessAndPublicSectorMember
1266	Consumer [Member]	Member			bt:ConsumerMember
1267	EE [Member]	Member			bt:EEMember
1268	Wholesale and Ventures [Member]	Member			bt:WholesaleAndVenturesMember
1269	Disclosure of information for cash-generating units [Line Items]	LineItems			ifrs-full:DisclosureOfInformationForIndividualAssetOrCashgeneratingUnitWithSignificantAmountOfGoodwillOrIntangibleAssetsWithIndefiniteUsefulLivesLineItems
1270	Pre-tax discount rate	Concept (Percent)	As Of		bt:PreTaxDiscountRate
1271	Perpetuity growth rate	Concept (Percent)	As Of		ifrs-full:DescriptionOfGrowthRateUsedToExtrapolateCashFlowProjections
1272	Amount by which unit's recoverable amount exceeds its carrying amount	Concept (Monetary)	As Of	Debit	ifrs-full:AmountByWhichUnitsRecoverableAmountExceedsItsCarryingAmount
1273	Sensitivity analysis, increase in discount rate	Concept (Percent)	As Of		bt:PercentageOfSensitivityAnalysisIcreaseInDiscountRate
1274	Sensitivity analysis of reduction in perpetuity growth rate revised assumption	Concept (Percent)	As Of		bt:PercentageOfSensitivityAnalysisRevisedAssumptionReductionInPerpetuityGrowthRate
1275	Sensitivity analysis, decrease in cash flows projection	Concept (Percent)	As Of		bt:PercentageOfSensitivityAnalysisDecreaseInOperatingCashFlows
1276	101130 - Disclosure - Property, Plant and Equipment - Summary of the Carrying Values of Software, Property, Plant and Equipment (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosurePropertyPlantAndEquipmentSummaryOfCarryingValuesOfSoftwarePropertyPlantAndEquipmentDetail
1277	Disclosure Of Property Plant And Equipment [Table]	Table	*	*	ifrs-full:DisclosureOfPropertyPlantAndEquipmentTable
1278	Classes of property, plant and equipment [axis]	Axis			ifrs-full:ClassesOfPropertyPlantAndEquipmentAxis
1279	Property, plant and equipment [member]	Member			ifrs-full:PropertyPlantAndEquipmentMember
1280	Land and Buildings [Member]	Member			ifrs-full:LandAndBuildingsMember
1281	Network Infrastructure [Member]	Member			ifrs-full:NetworkInfrastructureMember
1282	Other [Member]	Member			ifrs-full:OtherPropertyPlantAndEquipmentMember
1283	Assets in Course of Construction [Member]	Member			ifrs-full:ConstructionInProgressMember
1284	Classes of assets [axis]	Axis			ifrs-full:ClassesOfAssetsAxis
1285	Assets [member]	Member			ifrs-full:ClassesOfAssetsMember
1286	Engineering Stores [Member]	Member			bt:EngineeringStoresMember
1287	Carrying amount, accumulated depreciation, amortisation and impairment and gross carrying amount [axis]	Axis			ifrs-full:CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis
1288	Carrying amount [member]	Member			ifrs-full:CarryingAmountMember
1289	Gross Carrying Amount [Member]	Member			ifrs-full:GrossCarryingAmountMember
1290	Accumulated Depreciation and Amortisation [Member]	Member			ifrs-full:AccumulatedDepreciationAndAmortisationMember
1291	Disclosure of detailed information about property, plant and equipment [Line Items]	LineItems			ifrs-full:DisclosureOfPropertyPlantAndEquipmentLineItems
1292	Opening balance	Concept (Monetary)	As Of	Debit	ifrs-full:PropertyPlantAndEquipment
1293	Ending balance	Concept (Monetary)	As Of	Debit	bt:PropertyPlantAndEquipmentExcludingEngineeringStores
1294	Charge for the year	Concept (Monetary)	For Period		ifrs-full:DepreciationPropertyPlantAndEquipment
1295	Engineering stores	Concept (Monetary)	As Of	Debit	ifrs-full:ConstructionInProgress
1296	Additions	Concept (Monetary)	For Period	Debit	ifrs-full:AdditionsOtherThanThroughBusinessCombinationsPropertyPlantAndEquipment
1297	Transfers	Concept (Monetary)	For Period	Debit	ifrs-full:IncreaseDecreaseThroughTransfersPropertyPlantAndEquipment
1298	Disposals and adjustments	Concept (Monetary)	For Period	Credit	ifrs-full:DisposalsAndRetirementsPropertyPlantAndEquipment
1299	Exchange differences	Concept (Monetary)	For Period	Debit	ifrs-full:IncreaseDecreaseThroughNetExchangeDifferencesPropertyPlantAndEquipment
1300	Ending balance	Concept (Monetary)	As Of	Debit	ifrs-full:PropertyPlantAndEquipment
1301	101140 - Disclosure - Property, Plant and Equipment - Summary of the Carrying Values of Software, Property, Plant and Equipment (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosurePropertyPlantAndEquipmentSummaryOfCarryingValuesOfSoftwarePropertyPlantAndEquipmentParentheticalDetail
1302	Disclosure Of Property Plant And Equipment [Table]	Table	*	*	ifrs-full:DisclosureOfPropertyPlantAndEquipmentTable
1303	Classes of property, plant and equipment [axis]	Axis			ifrs-full:ClassesOfPropertyPlantAndEquipmentAxis
1304	Property, plant and equipment [member]	Member			ifrs-full:PropertyPlantAndEquipmentMember
1305	Land and Buildings [Member]	Member			ifrs-full:LandAndBuildingsMember
1306	Network Infrastructure [Member]	Member			ifrs-full:NetworkInfrastructureMember
1307	Disclosure of detailed information about property, plant and equipment [Line Items]	LineItems			ifrs-full:DisclosureOfPropertyPlantAndEquipmentLineItems
1308	Carrying amount of assets held under finance leases	Concept (Monetary)	As Of	Debit	bt:AssetsHeldUnderFinanceLeaseCarryingAmount
1309	Depreciation on assets held under finance leases	Concept (Monetary)	For Period	Debit	bt:DepreciationAssetsHeldUnderFinanceLeases
1310	Net grant deferral/funding	Concept (Monetary)	For Period	Debit	bt:NetGrantDeferralFunding
1311	Write off of fully depreciated assets	Concept (Monetary)	For Period	Debit	bt:WriteOffOfFullyDepreciatedAssets
1312	101150 - Disclosure - Property, Plant and Equipment - Summary of Property, Plant and Equipment (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosurePropertyPlantAndEquipmentSummaryOfPropertyPlantAndEquipmentDetail
1313	Disclosure Of Property Plant And Equipment [Table]	Table	*	*	ifrs-full:DisclosureOfPropertyPlantAndEquipmentTable
1314	Classes of property, plant and equipment [axis]	Axis			ifrs-full:ClassesOfPropertyPlantAndEquipmentAxis
1315	Property, plant and equipment [member]	Member			ifrs-full:PropertyPlantAndEquipmentMember
1316	Freehold [Member]	Member			bt:FreeholdsLandAndBuildingsMember
1317	Leasehold [Member]	Member			bt:LeaseholdLandAndBuildingsMember
1318	Disclosure of detailed information about property, plant and equipment [Line Items]	LineItems			ifrs-full:DisclosureOfPropertyPlantAndEquipmentLineItems
1319	Total land and buildings	Concept (Monetary)	As Of	Debit	ifrs-full:LandAndBuildings
1320	101160 - Disclosure - Property, Plant and Equipment - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosurePropertyPlantAndEquipmentAdditionalInformationDetail
1321	Disclosure Of Property Plant And Equipment [Table]	Table	*	*	ifrs-full:DisclosureOfPropertyPlantAndEquipmentTable
1322	Classes of property, plant and equipment [axis]	Axis			ifrs-full:ClassesOfPropertyPlantAndEquipmentAxis
1323	Property, plant and equipment [member]	Member			ifrs-full:PropertyPlantAndEquipmentMember
1324	Network Infrastructure [Member]	Member			ifrs-full:NetworkInfrastructureMember
1325	Geographical areas [axis]	Axis			ifrs-full:GeographicalAreasAxis
1326	Geographical areas [member]	Member			ifrs-full:GeographicalAreasMember
1327	UK [Member]	Member			country:GB
1328	Disclosure of detailed information about property, plant and equipment [Line Items]	LineItems			ifrs-full:DisclosureOfPropertyPlantAndEquipmentLineItems
1329	Net book value of share of assets owned by joint ventures	Concept (Monetary)	As Of	Debit	bt:NetBookValueOfSharesOfAssetsInJointVentures
1330	Value of share of assets owned	Concept (Monetary)	As Of	Debit	bt:ValueOfShareOfAssetsOwnedInAssociatesAndJointVentures
1331	Net book value of assets	Concept (Monetary)	As Of	Debit	bt:NetBookValueOfAssets
1332	Useful life of assets	Concept (Text/String)	For Period		ifrs-full:UsefulLivesOrDepreciationRatesPropertyPlantAndEquipment
1333	Net grant deferral/funding	Concept (Monetary)	For Period	Debit	bt:NetGrantDeferralFunding
1334	BDUK take-up assumption rate	Concept (Percent)	As Of		bt:BroadbandDeliveryTakeUpAssumptionRate
1335	Percentage of change in take up assumption rate	Concept (Percent)	For Period		bt:PercentageOfChangeInTakeUpAssumptionRate
1336	Movement in deferred grant funding based on change in take up assumption rate	Concept (Monetary)	For Period	Debit	bt:MovementInDeferredGrantFundingBasedOnChangeInTakeUpAssumptionRate
1337	Deferred grant funding	Concept (Monetary)	As Of	Debit	bt:DeferredGrantFunding
1338	101170 - Disclosure - Programme Rights - Summary of Programme Rights (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureProgrammeRightsSummaryOfProgrammeRightsDetail
1339	Disclosure Of Intangible Assets [Table]	Table	*	*	ifrs-full:DisclosureOfIntangibleAssetsTable
1340	Classes of intangible assets other than goodwill [axis]	Axis			ifrs-full:ClassesOfIntangibleAssetsOtherThanGoodwillAxis
1341	Intangible assets other than goodwill [member]	Member			ifrs-full:IntangibleAssetsOtherThanGoodwillMember
1342	Programme Rights [Member]	Member			bt:ProgrammeRightsMember
1343	Disclosure of detailed information about intangible assets [Line Items]	LineItems			ifrs-full:DisclosureOfIntangibleAssetsLineItems
1344	Beginning balance	Concept (Monetary)	As Of	Debit	ifrs-full:CopyrightsPatentsAndOtherIndustrialPropertyRightsServiceAndOperatingRights
1345	Additions	Concept (Monetary)	For Period	Debit	ifrs-full:AdditionsOtherThanThroughBusinessCombinationsIntangibleAssetsOtherThanGoodwill
1346	Amortisation	Concept (Monetary)	For Period		ifrs-full:AmortisationIntangibleAssetsOtherThanGoodwill
1347	Ending balance	Concept (Monetary)	As Of	Debit	ifrs-full:CopyrightsPatentsAndOtherIndustrialPropertyRightsServiceAndOperatingRights
1348	101180 - Disclosure - Inventories - Summary of Inventories (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureInventoriesSummaryOfInventoriesDetail
1349	Implied Table	Table	*	*	implied:Table
1350	Classes Of Inventories [Abstract]	Abstract			ifrs-full:ClassesOfInventoriesAbstract
1351	Consumables	Concept (Monetary)	As Of	Debit	ifrs-full:RawMaterials
1352	Work in progress	Concept (Monetary)	As Of	Debit	ifrs-full:WorkInProgress
1353	Finished goods	Concept (Monetary)	As Of	Debit	ifrs-full:FinishedGoods
1354	Inventories	Concept (Monetary)	As Of	Debit	ifrs-full:Inventories
1355	101190 - Disclosure - Inventories - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureInventoriesAdditionalInformationDetail
1356	Implied Table	Table	*	*	implied:Table
1357	Classes Of Inventories [Abstract]	Abstract			ifrs-full:ClassesOfInventoriesAbstract
1358	Inventories expenses	Concept (Monetary)	For Period	Debit	ifrs-full:CostOfInventoriesRecognisedAsExpenseDuringPeriod
1359	101200 - Disclosure - Trade and Other Receivables - Schedule of Trade and Other Receivables (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTradeAndOtherReceivablesScheduleOfTradeAndOtherReceivablesDetail
1360	Implied Table	Table	*	*	implied:Table
1361	Disclosure Of Trade And Other Receivables [Abstract]	Abstract			bt:DisclosureOfTradeAndOtherReceivablesAbstract
1362	Non-current	Abstract			ifrs-full:NoncurrentReceivablesAbstract
1363	Other assets	Concept (Monetary)	As Of	Debit	ifrs-full:NoncurrentReceivables
1364	Current	Abstract			ifrs-full:TradeAndOtherCurrentReceivablesAbstract
1365	Trade receivables	Concept (Monetary)	As Of	Debit	ifrs-full:CurrentTradeReceivables
1366	Prepayments	Concept (Monetary)	As Of	Debit	ifrs-full:CurrentPrepayments
1367	Accrued income	Concept (Monetary)	As Of	Debit	ifrs-full:CurrentAccruedIncome
1368	Other receivables	Concept (Monetary)	As Of	Debit	ifrs-full:OtherCurrentReceivables
1369	Trade and other receivable current	Concept (Monetary)	As Of	Debit	ifrs-full:TradeAndOtherCurrentReceivables
1370	101210 - Disclosure - Trade and Other Receivables - Schedule of Trade and Other Receivables (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTradeAndOtherReceivablesScheduleOfTradeAndOtherReceivablesParentheticalDetail
1371	Disclosure Of Trade And Other Receivables [Table]	Table	*	*	bt:DisclosureOfTradeAndOtherReceivablesTable
1372	Assets and liabilities classified as held for sale [axis]	Axis			ifrs-full:AssetsAndLiabilitiesClassifiedAsHeldForSaleAxis
1373	Assets and liabilities not classified as held for sale [member]	Member			ifrs-full:AssetsAndLiabilitiesNotClassifiedAsHeldForSaleMember
1374	Current Assets Held for Sale [Member]	Member			bt:CurrentAssetsHeldForSaleMember
1375	Business combinations [axis]	Axis			ifrs-full:BusinessCombinationsAxis
1376	Entity's total for business combinations [member]	Member			ifrs-full:EntitysTotalForBusinessCombinationsMember
1377	Spectrum [Member]	Member			bt:SpectrumMember
1378	Disclosure of Trade and Other Receivables [line items]	LineItems			bt:DisclosureOfTradeAndOtherReceivablesLineItems
1379	Other assets relating to initial set-up, transition or transformation phase of long-term networked IT services contracts	Concept (Monetary)	As Of	Debit	ifrs-full:OtherNoncurrentReceivables
1380	Other assets relating to prepayments and leasing debtors	Concept (Monetary)	For Period	Debit	bt:PrepaymentsAndLeasingDebtors
1381	Prepayments in respect of acquisition	Concept (Monetary)	As Of	Debit	bt:PrepaymentsInRespectOfAcquisition
1382	Other receivables	Concept (Monetary)	As Of	Debit	ifrs-full:OtherCurrentReceivables
1383	Acquisition cost	Concept (Monetary)	For Period	Debit	bt:AcquisitionCost
1384	101220 - Disclosure - Trade and Other Receivables - Summary of Trade Receivables After Deducting Allowances for Doubtful Debts (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTradeAndOtherReceivablesSummaryOfTradeReceivablesAfterDeductingAllowancesForDoubtfulDebtsDetail
1385	Implied Table	Table	*	*	implied:Table
1386	Trade And Other Receivables [Abstract]	Abstract			ifrs-full:TradeAndOtherReceivablesAbstract
1387	At 1 April	Concept (Monetary)	As Of	Credit	ifrs-full:AllowanceAccountForCreditLossesOfFinancialAssets
1388	Expense	Concept (Monetary)	For Period		ifrs-full:AdditionalAllowanceRecognisedInProfitOrLossAllowanceAccountForCreditLossesOfFinancialAssets
1389	Utilised	Concept (Monetary)	For Period	Debit	ifrs-full:UtilisationAllowanceAccountForCreditLossesOfFinancialAssets
1390	Exchange differences	Concept (Monetary)	For Period	Credit	ifrs-full:IncreaseDecreaseThroughNetExchangeDifferencesAllowanceAccountForCreditLossesOfFinancialAssets
1391	At 31 March	Concept (Monetary)	As Of	Credit	ifrs-full:AllowanceAccountForCreditLossesOfFinancialAssets
1392	101230 - Disclosure - Trade and Other Receivables - Summary of Trade Receivables Past Due and Not Specifically Impaired (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTradeAndOtherReceivablesSummaryOfTradeReceivablesPastDueAndNotSpecificallyImpairedDetail
1393	Disclosure Of Trade And Other Receivables [Table]	Table	*	*	bt:DisclosureOfTradeAndOtherReceivablesTable
1394	Past due status [axis]	Axis			ifrs-full:PastDueStatusAxis
1395	Past due status [member]	Member			ifrs-full:PastDueStatusMember
1396	Current [Member]	Member			ifrs-full:CurrentMember
1397	Between 0 and 3 Months [Member]	Member			ifrs-full:NotLaterThanThreeMonthsMember
1398	Between 3 and 6 Months [Member]	Member			ifrs-full:LaterThanThreeMonthsAndNotLaterThanSixMonthsMember
1399	Between 6 and 12 Months [Member]	Member			ifrs-full:LaterThanSixMonthsAndNotLaterThanOneYearMember
1400	Over 12 Months [Member]	Member			ifrs-full:LaterThanOneYearMember
1401	Disclosure of Trade and Other Receivables [line items]	LineItems			bt:DisclosureOfTradeAndOtherReceivablesLineItems
1402	Trade receivables	Concept (Monetary)	As Of	Debit	ifrs-full:CurrentTradeReceivables
1403	Trade receivables specifically impaired net of provision	Concept (Monetary)	As Of	Debit	bt:TradeReceivablesSpecificallyImpairedNetOfProvision
1404	101240 - Disclosure - Trade and Other Receivables - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTradeAndOtherReceivablesAdditionalInformationDetail
1405	Disclosure Of Trade And Other Receivables [Table]	Table	*	*	bt:DisclosureOfTradeAndOtherReceivablesTable
1406	Impairment of financial assets [axis]	Axis			ifrs-full:ImpairmentOfFinancialAssetsAxis
1407	Entity's total for impairment of financial assets [member]	Member			ifrs-full:EntitysTotalForImpairmentOfFinancialAssetsMember
1408	Financial Assets Impaired [Member]	Member			ifrs-full:FinancialAssetsImpairedMember
1409	Disclosure of Trade and Other Receivables [line items]	LineItems			bt:DisclosureOfTradeAndOtherReceivablesLineItems
1410	Gross trade receivables specifically impaired	Concept (Monetary)	As Of	Debit	ifrs-full:TradeReceivables
1411	Cash collateral held against trade and other receivables	Concept (Monetary)	As Of	Credit	bt:CashCollateralHeldAgainstTradeAndOtherReceivables
1412	101250 - Disclosure - Trade and Other Receivables - Schedule of Trade Receivables Not Past Due and Accrued Income (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTradeAndOtherReceivablesScheduleOfTradeReceivablesNotPastDueAndAccruedIncomeDetail
1413	Disclosure Of Trade And Other Receivables [Table]	Table	*	*	bt:DisclosureOfTradeAndOtherReceivablesTable
1414	Segments [axis]	Axis			ifrs-full:SegmentsAxis
1415	Segments [member]	Member			ifrs-full:SegmentsMember
1416	BT Consumer [Member]	Member			bt:BTConsumerMember
1417	EE [Member]	Member			bt:EELimitedMember
1418	Business and Public Sector [Member]	Member			bt:BusinessAndPublicSectorSegmentMember
1419	Global Services [Member]	Member			bt:GlobalServicesSegmentMember
1420	Wholesale and Ventures [Member]	Member			bt:WholesaleAndVenturesSegmentMember
1421	Openreach [Member]	Member			bt:OpenreachSegmentMember
1422	Other [Member]	Member			ifrs-full:AllOtherSegmentsMember
1423	Past due status [axis]	Axis			ifrs-full:PastDueStatusAxis
1424	Past due status [member]	Member			ifrs-full:PastDueStatusMember
1425	Current [Member]	Member			ifrs-full:CurrentMember
1426	Disclosure of Trade and Other Receivables [line items]	LineItems			bt:DisclosureOfTradeAndOtherReceivablesLineItems
1427	Trade Receivables Not Past Due	Concept (Monetary)	As Of	Debit	ifrs-full:TradeReceivables
1428	Accrued income	Concept (Monetary)	As Of	Debit	ifrs-full:AccruedIncome
1429	101260 - Disclosure - Trade and Other Payables - Summary of Trade and Other Payables (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTradeAndOtherPayablesSummaryOfTradeAndOtherPayablesDetail
1430	Implied Table	Table	*	*	implied:Table
1431	Trade And Other Payables [Abstract]	Abstract			ifrs-full:TradeAndOtherPayablesAbstract
1432	Current	Abstract			ifrs-full:TradeAndOtherCurrentPayablesAbstract
1433	Trade payables	Concept (Monetary)	As Of	Credit	ifrs-full:TradeAndOtherCurrentPayablesToTradeSuppliers
1434	Other taxation and social security	Concept (Monetary)	As Of	Credit	ifrs-full:CurrentPayablesOnSocialSecurityAndTaxesOtherThanIncomeTax
1435	Other payables	Concept (Monetary)	As Of	Credit	ifrs-full:OtherCurrentPayables
1436	Accrued expenses	Concept (Monetary)	As Of	Credit	ifrs-full:AccrualsClassifiedAsCurrent
1437	Deferred income	Concept (Monetary)	As Of	Credit	ifrs-full:DeferredIncomeClassifiedAsCurrent
1438	Total current trade and other payables	Concept (Monetary)	As Of	Credit	ifrs-full:TradeAndOtherCurrentPayables
1439	Non-current	Abstract			ifrs-full:NoncurrentPayablesAbstract
1440	Other payables	Concept (Monetary)	As Of	Credit	ifrs-full:OtherNoncurrentPayables
1441	Deferred income	Concept (Monetary)	As Of	Credit	ifrs-full:DeferredIncomeClassifiedAsNoncurrent
1442	Total non-current trade and other payables	Concept (Monetary)	As Of	Credit	ifrs-full:NoncurrentPayables
1443	101270 - Disclosure - Trade and Other Payables - Summary of Trade and Other Payables (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureTradeAndOtherPayablesSummaryOfTradeAndOtherPayablesParentheticalDetail
1444	Disclosure Of Trade And Other Payables [Table]	Table	*	*	bt:DisclosureOfTradeAndOtherPayablesTable
1445	Government Grants	Axis			bt:GovernmentGrantsAxis
1446	Government Grants Member	Member			bt:GovernmentGrantsMember
1447	Broadband Delivery UK Programme [Member]	Member			bt:BroadbandDeliveryUKProgrammeMember
1448	Disclosure of trade and other payables [Line Items]	LineItems			bt:DisclosureOfTradeAndOtherPayablesLineItems
1449	Deferred income current	Concept (Monetary)	As Of	Credit	ifrs-full:DeferredIncomeClassifiedAsCurrent
1450	Deferred income non-current	Concept (Monetary)	As Of	Credit	ifrs-full:DeferredIncomeClassifiedAsNoncurrent
1451	101280 - Disclosure - Provisions - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureProvisionsAdditionalInformationDetail
1452	Disclosure Of Other Provisions [Table]	Table	*	*	ifrs-full:DisclosureOfOtherProvisionsTable
1453	Retrospective application and retrospective restatement [axis]	Axis			ifrs-full:RetrospectiveApplicationAndRetrospectiveRestatementAxis
1454	Currently stated [member]	Member			ifrs-full:RestatedMember
1455	Specific Item [Member]	Member			bt:SpecificItemMember
1456	Classes of other provisions [axis]	Axis			ifrs-full:ClassesOfProvisionsAxis
1457	Other provisions [member]	Member			ifrs-full:OtherProvisionsMember
1458	Regulatory [Member]	Member			bt:RegulatoryMember
1459	Fine [Member]	Member			bt:FineOneMember
1460	Fine and Associated Compensation Payments [Member]	Member			bt:ProvisionMember
1461	Disclosure of other provisions [Line Items]	LineItems			ifrs-full:DisclosureOfOtherProvisionsLineItems
1462	Additional provisions	Concept (Monetary)	For Period	Credit	ifrs-full:IncreaseDecreaseInExistingProvisionsOtherProvisions
1463	Trade and other payables	Concept (Monetary)	As Of	Credit	ifrs-full:TradeAndOtherCurrentPayables
1464	Operating costs	Concept (Monetary)	For Period	Debit	ifrs-full:OperatingExpense
1465	Contract loss provisions	Concept (Monetary)	For Period	Debit	bt:ProvisionForLossContract
1466	101290 - Disclosure - Provisions - Details of Estimate of the Provisions (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureProvisionsDetailsOfEstimateOfProvisionsDetail
1467	Disclosure Of Other Provisions [Table]	Table	*	*	ifrs-full:DisclosureOfOtherProvisionsTable
1468	Classes of other provisions [axis]	Axis			ifrs-full:ClassesOfProvisionsAxis
1469	Other provisions [member]	Member			ifrs-full:OtherProvisionsMember
1470	Restructuring Provision [Member]	Member			ifrs-full:RestructuringProvisionMember
1471	Property, plant and equipment [member]	Member			ifrs-full:PropertyPlantAndEquipmentMember
1472	Network ARO Provision [Member]	Member			bt:NetworkAroProvisionMember
1473	Network Share Provision [Member]	Member			bt:NetworkShareProvisionMember
1474	Regulatory [Member]	Member			bt:RegulatoryMember
1475	Litigation [Member]	Member			ifrs-full:LegalProceedingsProvisionMember
1476	Restructuring Provision [Member]	Member			ifrs-full:MiscellaneousOtherProvisionsMember
1477	Disclosure of other provisions [Line Items]	LineItems			ifrs-full:DisclosureOfOtherProvisionsLineItems
1478	Opening balance	Concept (Monetary)	As Of	Credit	ifrs-full:Provisions
1479	Income statement expense	Concept (Monetary)	For Period	Credit	ifrs-full:IncreaseDecreaseInExistingProvisionsOtherProvisions
1480	Unwind of discount	Concept (Monetary)	For Period	Debit	ifrs-full:ExpenseDueToUnwindingOfDiscountOnProvisions
1481	Utilised or released	Concept (Monetary)	For Period	Debit	ifrs-full:ProvisionUsedOtherProvisions
1482	Transfers	Concept (Monetary)	For Period	Credit	ifrs-full:ChangesInOtherProvisions
1483	Exchange differences	Concept (Monetary)	For Period	Credit	ifrs-full:IncreaseDecreaseThroughNetExchangeDifferencesOtherProvisions
1484	Ending balance	Concept (Monetary)	As Of	Credit	ifrs-full:Provisions
1485	101300 - Disclosure - Provisions - Details of Provisions (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureProvisionsDetailsOfProvisionsDetail
1486	Implied Table	Table	*	*	implied:Table
1487	Disclosure Of Other Provisions [Abstract]	Abstract			ifrs-full:DisclosureOfOtherProvisionsAbstract
1488	Current	Concept (Monetary)	As Of	Credit	ifrs-full:CurrentProvisions
1489	Non-current	Concept (Monetary)	As Of	Credit	ifrs-full:NoncurrentProvisions
1490	Total	Concept (Monetary)	As Of	Credit	ifrs-full:Provisions
1491	101310 - Disclosure - Retirement Benefit Plans - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlansAdditionalInformationDetail
1492	Disclosure Of Defined Benefit Plans [Table]	Table	*	*	ifrs-full:DisclosureOfDefinedBenefitPlansTable
1493	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
1494	Defined benefit plans [member]	Member			ifrs-full:DefinedBenefitPlansMember
1495	BT Pension Scheme [Member]	Member			bt:BtPensionSchemeMember
1496	BT Retirement Saving Scheme [Member]	Member			bt:BTRetirementSavingSchemeMember
1497	BTPS [Member]	Member			bt:BTPSMember
1498	Funding Contribution [Member]	Member			bt:FundingContributionMember
1499	Ordinary Contributions [Member]	Member			bt:OrdinaryContributionsMember
1500	Deficit Contributions [Member]	Member			bt:DeficitContributionsMember
1501	EEPS [Member]	Member			bt:EEPSMember
1502	EE Pension Scheme [Member]	Member			bt:EEPensionSchemeMember
1503	Classes of financial instruments [axis]	Axis			ifrs-full:ClassesOfFinancialInstrumentsAxis
1504	Financial instruments, class [member]	Member			ifrs-full:ClassesOfFinancialInstrumentsMember
1505	Trade and Other Payables [Member]	Member			bt:TradeAndOtherPayableMember
1506	Range [axis]	Axis			ifrs-full:RangeAxis
1507	Ranges [member]	Member			ifrs-full:RangesMember
1508	Top of Range [Member]	Member			ifrs-full:TopOfRangeMember
1509	Bottom of Range [Member]	Member			ifrs-full:BottomOfRangeMember
1510	Performance obligations [axis]	Axis			ifrs-full:PerformanceObligationsAxis
1511	Performance obligations [member]	Member			ifrs-full:PerformanceObligationsMember
1512	Over Three Years to 31 March 2020 [Member]	Member			bt:OverThreeYearsToThirtyFirstMarchTwentyTwentyMember
1513	Non-adjusting events after reporting period [axis]	Axis			ifrs-full:NonadjustingEventsAfterReportingPeriodAxis
1514	Non-adjusting events after reporting period [member]	Member			ifrs-full:NonadjustingEventsMember
1515	Events After Reporting Period [Member]	Member			bt:EventsAfterReportingPeriodMember
1516	Net defined benefit liability (asset) [axis]	Axis			ifrs-full:NetDefinedBenefitLiabilityAssetAxis
1517	Net defined benefit liability (asset) [member]	Member			ifrs-full:NetDefinedBenefitLiabilityAssetMember
1518	Global Equities [Member]	Member			bt:GlobalEquitiesMember
1519	Property and Illiquid Alternatives [Member]	Member			bt:PropertyAndIlliquidAlternativesMember
1520	Absolute Return Portfolio [Member]	Member			bt:AbsoluteReturnPortfolioMember
1521	Liability Driven Investment Portfolio [Member]	Member			bt:LiabilityDrivenInvestmentPortfolioMember
1522	Maturity [axis]	Axis			ifrs-full:MaturityAxis
1523	Aggregated time bands [member]	Member			ifrs-full:AggregatedTimeBandsMember
1524	Each Month Between 1 April 2018 and November 2020	Member			bt:EachMonthBetweenFirstAprilTwoThousandEighteenAndNovemberTwentyTwentyMember
1525	Defined Contribution Plans	Axis			bt:DefinedContributionPlansAxis
1526	Defined Contribution Plans	Member			bt:DefinedContributionPlansDomain
1527	BTRSS [Member]	Member			bt:BtrssMember
1528	Other Pension Plans [Member]	Member			bt:OtherPensionPlansMember
1529	Geographical areas [axis]	Axis			ifrs-full:GeographicalAreasAxis
1530	Geographical areas [member]	Member			ifrs-full:GeographicalAreasMember
1531	Netherlands [Member]	Member			country:NL
1532	Disclosure Of Retirement Benefit Plans [Line Items]	LineItems			bt:DisclosureOfRetirementBenefitPlansLineItems
1533	Percentage of future benefit accrual	Concept (Percent)	For Period		bt:PercentageOfFutureBenefitAccrual
1534	Present value of liabilities	Concept (Monetary)	For Period	Debit	bt:ContributionsPayableToDefinedContributionPlans
1535	Contribution rate	Concept (Percent)	For Period		bt:PercentageOfContributionsToPlanByEmployerNetDefinedBenefitLiabilityAsset
1536	Transition payments percentage	Concept (Percent)	For Period		bt:TransitionPaymentsPercentage
1537	Number of members in pension scheme	Concept (Decimal)	As Of		bt:NumberOfMembersIncludedInPensionScheme
1538	Weighted average term of liabilities	Concept (xbrli:durationItemType)	For Period		bt:WeightedAverageTermOfLiabilities
1539	Length of time over which payments will be made to all retirees from the fund	Concept (xbrli:durationItemType)	For Period		bt:LengthOfTimeOverWhichPaymentsWillBeMadeToAllRetireesFromTheFund
1540	Normal pensionable age	Concept (xbrli:durationItemType)	For Period		bt:NormalPensionableAge
1541	Percentage of group's retirement benefit obligation	Concept (Percent)	As Of		bt:PercentageOfRetirementBenefitObligation
1542	Percentage retirement benefit obligation are rounded	Concept (Percent)	For Period		bt:PercentageRetirementBenefitObligationAreRounded
1543	Percentage of exposure covered by longevity insurance	Concept (Percent)	For Period		bt:PercentageOfExposureCoveredByLongevityInsurance
1544	Maximum period of addressing deficit	Concept (xbrli:durationItemType)	For Period		bt:MaximumPeriodOfAddressingTheDeficit
1545	Number of recent triennial valuations	Concept (Integer)	For Period		bt:NumberOfRecentTriennialValuations
1546	Deficit contributions	Concept (Monetary)	As Of	Credit	bt:FundingDeficitContribution
1547	Asset return percentage	Concept (Percent)	For Period		bt:AssetReturnPercentage
1548	Percentage of investment in growth asset	Concept (Percent)	As Of		bt:PercentageOfInvestmentInGrowthAsset
1549	De-risking to low risk investment approach year	Concept (Year)	For Period		bt:DeRiskingToLowRiskInvestmentApproachYear
1550	Prudent discount rate	Concept (Percent)	As Of		bt:PrudentDiscountRate
1551	Flat discount rate	Concept (Percent)	As Of		bt:FlatDiscountRate
1552	Funding deficit	Concept (Monetary)	As Of	Credit	bt:FundingDeficit
1553	Funding deficit term	Concept (xbrli:durationItemType)	For Period		bt:FundingDeficitRecoveryPlanTerm
1554	Payments under recovery plan	Concept (Monetary)	For Period	Credit	bt:PaymentsUnderRecoveryPlan
1555	Deficit contributions paid	Concept (Monetary)	For Period	Debit	bt:DeficitContributionsPaid
1556	Remaining deficit contributionspPaid	Concept (Monetary)	For Period	Debit	bt:RemainingDeficitContributionsPaid
1557	Additional contribution due to be funded from proceeds of issuance of bonds	Concept (Monetary)	For Period	Credit	bt:AdditionalContributionDueToBeFundedFromProceedsOfIssuanceOfBonds
1558	Bond maturity term	Concept (Year)	For Period		bt:BondMaturityTerm
1559	Fixed interest on bonds	Concept (Monetary)	For Period	Debit	ifrs-full:InterestExpenseOnBonds
1560	CPI Linked Bonds	Concept (Monetary)	For Period	Debit	bt:CPILinkedBonds
1561	Annual payment term	Concept (Date)	For Period		bt:AnnualPaymentTerm
1562	Annual payment amount	Concept (Monetary)	For Period	Debit	bt:AnnualPaymentAmount
1563	Material Contributions	Concept (Monetary)	For Period	Debit	bt:MaterialContributions
1564	Group contributions based on funding valuation agreement	Concept (Monetary)	For Period	Debit	bt:GroupContributionsBasedOnFundingValuationAgreement
1565	Threshold dividend per share	Concept (Percent)	For Period		bt:ThresholdDividendPerShare
1566	Threshold dividend per share of share buybacks	Concept (Monetary)	For Period	Debit	bt:ThresholdDividendPerShareOfShareBuybacks
1567	Share buybacks in excess per year	Concept (Monetary)	For Period	Debit	bt:NotionalThresholdForBuybacksOverWhichPensionTrusteesConsulted
1568	Proceeds from disposals net of acquisitions	Concept (Monetary)	For Period	Debit	ifrs-full:ProceedsFromDisposalsOfPropertyPlantAndEquipmentIntangibleAssetsOtherThanGoodwillInvestmentPropertyAndOtherNoncurrentAssets
1569	Acquisition Consideration	Concept (Monetary)	For Period	Credit	bt:AcquisitionConsiderationUnderMaterialCorporateEvents
1570	Disposal Consideration	Concept (Monetary)	For Period	Debit	bt:DisposalConsiderationUnderMaterialCorporateEvents
1571	Threshold for BTPS	Concept (Monetary)	For Period	Debit	bt:ThresholdForBTPS
1572	Deficit threshold level for application for provision	Concept (Monetary)	For Period	Debit	bt:DeficitThresholdLevelForApplicationForProvision
1573	Defined benefit liabilities	Concept (Monetary)	As Of	Credit	bt:DefinedBenefitLiabilities
1574	Number of active members	Concept (Decimal)	For Period		bt:NumberOfActiveMembers
1575	Percentage of assets of benefit plan	Concept (Percent)	As Of		bt:PercentageOfAssetsOfBenefitPlan
1576	Monthly payments to bridge funding deficit	Concept (Monetary)	For Period	Credit	bt:MonthlyPaymentsToBridgeFundingDeficit
1577	Contribution to plan	Concept (Monetary)	For Period	Credit	ifrs-full:EmployerContributions
1578	Number of closed schemes	Concept (Integer)	For Period		bt:NumberOfClosedSchemes
1579	Past service credit	Concept (Monetary)	For Period	Credit	ifrs-full:PastServiceCostNetDefinedBenefitLiabilityAsset
1580	101320 - Disclosure - Retirement Benefit Plan - Summary of Retirement Benefit Plan (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlanSummaryOfRetirementBenefitPlanDetail
1581	Disclosure Of Defined Benefit Plans [Table]	Table	*	*	ifrs-full:DisclosureOfDefinedBenefitPlansTable
1582	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
1583	Defined benefit plans [member]	Member			ifrs-full:DefinedBenefitPlansMember
1584	Defined Contribution Plans [Member]	Member			bt:DefinedContributionPlansMember
1585	Defined Benefit Plan [Member]	Member			bt:DefinedBenefitPlanMember
1586	Disclosure Of Defined Benefit Plans [Line Items]	LineItems			ifrs-full:DisclosureOfDefinedBenefitPlansLineItems
1587	What are they?	Concept (Text/String)	For Period		ifrs-full:DescriptionOfRetirementBenefitsPromisedToParticipants
1588	How do they impact BT�s financial statements?	Concept (Text/String)	For Period		bt:DescriptionOfImpactOfRetirementBenefitPlanInFinancialStatements
1589	101330 - Disclosure - Retirement Benefit Plan - Summary of Expense or Income from Retirement Benefit Arrangements Recognised in Income Statement (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlanSummaryOfExpenseOrIncomeFromRetirementBenefitArrangementsRecognisedInIncomeStatementDetail
1590	Disclosure Of Defined Benefit Plans [Table]	Table	*	*	ifrs-full:DisclosureOfDefinedBenefitPlansTable
1591	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
1592	Defined benefit plans [member]	Member			ifrs-full:DefinedBenefitPlansMember
1593	Defined Benefit Plan [Member]	Member			bt:DefinedBenefitPlanMember
1594	Defined Contribution Plans [Member]	Member			bt:DefinedContributionPlansMember
1595	Past Service Credit [Member]	Member			bt:PastServiceCreditMember
1596	Disclosure Of Defined Benefit Plans [Line Items]	LineItems			ifrs-full:DisclosureOfDefinedBenefitPlansLineItems
1597	Service cost (including administration expenses & PPF levy):	Abstract			bt:ServiceCostAbstract
1598	Service cost (including administration expenses & PPF levy)	Concept (Monetary)	For Period	Credit	ifrs-full:CurrentServiceCostNetDefinedBenefitLiabilityAsset
1599	Net interest expense on pensions deficit included in specific items (note 8)	Concept (Monetary)	For Period	Credit	ifrs-full:InterestExpenseIncomeNetDefinedBenefitLiabilityAsset
1600	Total recognised in the income statement	Concept (Monetary)	For Period	Debit	bt:PostRetirementExpensesRecognizedInStatementOfIncome
1601	101340 - Disclosure - Retirement Benefit Plan - Summary of Pension Obligation in Respect of Defined Benefit Plans Reported in Balance Sheet (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlanSummaryOfPensionObligationInRespectOfDefinedBenefitPlansReportedInBalanceSheetDetail
1602	Disclosure Of Defined Benefit Plans [Table]	Table	*	*	ifrs-full:DisclosureOfDefinedBenefitPlansTable
1603	Net defined benefit liability (asset) [axis]	Axis			ifrs-full:NetDefinedBenefitLiabilityAssetAxis
1604	Net defined benefit liability (asset) [member]	Member			ifrs-full:NetDefinedBenefitLiabilityAssetMember
1605	Retirement Benefit Obligation Before Adjustments [Member]	Member			bt:RetirementBenefitObligationBeforeAdjustmentsMember
1606	Deferred Tax Asset [Member]	Member			bt:DeferredTaxAssetMember
1607	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
1608	Defined benefit plans [member]	Member			ifrs-full:DefinedBenefitPlansMember
1609	BT Pension Scheme [Member]	Member			bt:BtPensionSchemeMember
1610	EE Pension Scheme [Member]	Member			bt:EEPensionSchemeMember
1611	Other Pension Plans [Member]	Member			bt:OtherPensionPlansMember
1612	Disclosure Of Defined Benefit Plans [Line Items]	LineItems			ifrs-full:DisclosureOfDefinedBenefitPlansLineItems
1613	Assets	Concept (Monetary)	As Of	Debit	ifrs-full:PlanAssetsAtFairValue
1614	Present value of liabilities	Concept (Monetary)	As Of	Credit	ifrs-full:DefinedBenefitObligationAtPresentValue
1615	Deficit	Concept (Monetary)	As Of	Debit	ifrs-full:SurplusDeficitInPlan
1616	101350 - Disclosure - Retirement Benefit Plan - Summary of Pension Obligation in Respect of Defined Benefit Plans Reported in Balance Sheet (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlanSummaryOfPensionObligationInRespectOfDefinedBenefitPlansReportedInBalanceSheetParentheticalDetail
1617	Disclosure Of Defined Benefit Plans [Table]	Table	*	*	ifrs-full:DisclosureOfDefinedBenefitPlansTable
1618	Classes of financial liabilities [axis]	Axis			ifrs-full:ClassesOfFinancialLiabilitiesAxis
1619	Financial liabilities, class [member]	Member			ifrs-full:FinancialLiabilitiesMember
1620	Unfunded Pension Arrangements [Member]	Member			bt:UnfundedPensionArrangementsMember
1621	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
1622	Defined benefit plans [member]	Member			ifrs-full:DefinedBenefitPlansMember
1623	Other Plans [Member]	Member			bt:OtherPlansMember
1624	Disclosure Of Defined Benefit Plans [Line Items]	LineItems			ifrs-full:DisclosureOfDefinedBenefitPlansLineItems
1625	Present value of liabilities	Concept (Monetary)	As Of	Credit	ifrs-full:DefinedBenefitObligationAtPresentValue
1626	101360 - Disclosure - Retirement Benefit Plan - Summary of Movements on Pension Assets and Liabilities (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlanSummaryOfMovementsOnPensionAssetsAndLiabilitiesDetail
1627	Disclosure Of Defined Benefit Plans [Table]	Table	*	*	ifrs-full:DisclosureOfDefinedBenefitPlansTable
1628	Net defined benefit liability (asset) [axis]	Axis			ifrs-full:NetDefinedBenefitLiabilityAssetAxis
1629	Net defined benefit liability (asset) [member]	Member			ifrs-full:NetDefinedBenefitLiabilityAssetMember
1630	Assets [Member]	Member			ifrs-full:PlanAssetsMember
1631	Liabilities [Member]	Member			ifrs-full:PresentValueOfDefinedBenefitObligationMember
1632	Deficit [Member]	Member			bt:DeficitsMember
1633	Disclosure Of Defined Benefit Plans [Line Items]	LineItems			ifrs-full:DisclosureOfDefinedBenefitPlansLineItems
1634	Beginning balance	Concept (Monetary)	As Of	Credit	ifrs-full:LiabilityAssetOfDefinedBenefitPlans
1635	Service cost (including administration expenses and PPF levy)	Concept (Monetary)	For Period	Credit	ifrs-full:CurrentServiceCostNetDefinedBenefitLiabilityAsset
1636	Interest on pension deficit	Concept (Monetary)	For Period	Credit	ifrs-full:InterestExpenseIncomeNetDefinedBenefitLiabilityAsset
1637	Included in the group income statement	Concept (Monetary)	For Period	Debit	bt:GainLossOnMeasurementOfNetDefinedBenefitLiabilityAsset
1638	Return on plan assets included in the group income statement	Concept (Monetary)	For Period	Debit	ifrs-full:ReturnOnPlanAssetsNetDefinedBenefitLiabilityAsset
1639	Actuarial loss arising from changes in financial assumptions	Concept (Monetary)	For Period	Debit	ifrs-full:ActuarialGainsLossesArisingFromChangesInFinancialAssumptionsNetDefinedBenefitLiabilityAsset
1640	Actuarial loss arising from changes in demographic assumptions	Concept (Monetary)	For Period	Debit	ifrs-full:ActuarialGainsLossesArisingFromChangesInDemographicAssumptionsNetDefinedBenefitLiabilityAsset
1641	Actuarial gain arising from experience adjustments	Concept (Monetary)	For Period	Debit	bt:ActuarialGainsLossesArisingFromExperienceAdjustmentsNetDefinedBenefitLiabilityAsset
1642	Included in the group statement of comprehensive income	Concept (Monetary)	For Period	Debit	bt:ChangesInNetDefinedBenefitLiabilityAssetIncludedInGroupStatementOfComprehensiveIncome
1643	Regular contributions by employer	Concept (Monetary)	For Period	Debit	ifrs-full:ContributionsToPlanByEmployerNetDefinedBenefitLiabilityAsset
1644	Deficit contributions by employer	Concept (Monetary)	For Period	Debit	bt:DeficitContributionsToPlanByEmployerNetDefinedBenefitLiabilityAsset
1645	Included in the group cash flow statement	Concept (Monetary)	For Period	Debit	bt:ChangesInNetDefinedBenefitLiabilityAssetIncludedInGroupCashFowStatement
1646	Contributions by employees	Concept (Monetary)	For Period	Debit	ifrs-full:ContributionsToPlanByPlanParticipantsNetDefinedBenefitLiabilityAsset
1647	Benefits paid	Concept (Monetary)	For Period	Debit	ifrs-full:BenefitsPaidOrPayable
1648	Foreign exchange	Concept (Monetary)	For Period	Credit	ifrs-full:IncreaseDecreaseThroughChangesInForeignExchangeRatesNetDefinedBenefitLiabilityAsset
1649	Other movements	Concept (Monetary)	For Period	Credit	ifrs-full:IncreaseDecreaseThroughOtherChangesNetDefinedBenefitLiabilityAsset
1650	Ending balance	Concept (Monetary)	As Of	Credit	ifrs-full:LiabilityAssetOfDefinedBenefitPlans
1651	Past service credit	Concept (Monetary)	For Period	Credit	bt:PastServiceCreditDefinedBenefitLiabilityAsset
1652	101370 - Disclosure - Retirement Benefit Plan - Summary of Analysis of Membership (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlanSummaryOfAnalysisOfMembershipDetail
1653	Disclosure Of Defined Benefit Plans [Table]	Table	*	*	ifrs-full:DisclosureOfDefinedBenefitPlansTable
1654	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
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1656	BT Pension Scheme [Member]	Member			bt:BtPensionSchemeMember
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1658	Net defined benefit liability (asset) [member]	Member			ifrs-full:NetDefinedBenefitLiabilityAssetMember
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1661	Pensioners [Member]	Member			bt:PensionersMember
1662	Voluntary changes in accounting policy [axis]	Axis			ifrs-full:VoluntaryChangesInAccountingPolicyAxis
1663	Voluntary changes in accounting policy [member]	Member			ifrs-full:VoluntaryChangesInAccountingPolicyMember
1664	IAS 19 Sections A and B Liabilities [Member]	Member			bt:IASNineteenSectionsAAndBLiabilitiesMember
1665	IAS 19 Section C Liabilities [Member]	Member			bt:IASNineteenSectionCLiabilitiesMember
1666	Disclosure Of Defined Benefit Plans [Line Items]	LineItems			ifrs-full:DisclosureOfDefinedBenefitPlansLineItems
1667	IAS 19 liabilities (bn)	Concept (Monetary)	As Of	Credit	ifrs-full:LiabilityAssetOfDefinedBenefitPlans
1668	Total number of members	Concept (Decimal)	As Of		bt:NumberOfMembersIncludedInPensionScheme
1669	101380 - Disclosure - Retirement Benefit Plan - Summary of Fair Value of Assets of BTPS Analysed by Asset Category (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlanSummaryOfFairValueOfAssetsOfBTPSAnalysedByAssetCategoryDetail
1670	Disclosure Of Defined Benefit Plans [Table]	Table	*	*	ifrs-full:DisclosureOfDefinedBenefitPlansTable
1671	Measurement [axis]	Axis			ifrs-full:MeasurementAxis
1672	Aggregated measurement [member]	Member			ifrs-full:AggregatedMeasurementMember
1673	At Fair Value [Member]	Member			ifrs-full:AtFairValueMember
1674	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
1675	Defined benefit plans [member]	Member			ifrs-full:DefinedBenefitPlansMember
1676	BT Pension Scheme [Member]	Member			bt:BtPensionSchemeMember
1677	Assets and liabilities [axis]	Axis			ifrs-full:AssetsAndLiabilitiesAxis
1678	Assets and liabilities [member]	Member			ifrs-full:AssetsAndLiabilitiesMember
1679	Active Equity [Member]	Member			bt:ActiveEquityMember
1680	Passive Equity [Member]	Member			bt:PassiveEquityMember
1681	Private Equity [Member]	Member			bt:PrivateEquityMember
1682	UK Equity [Member]	Member			bt:UKEquitiesMember
1683	Overseas Developed Equity [Member]	Member			bt:OverseasDevelopedEquitiesMember
1684	Emerging Markets Equity [Member]	Member			bt:EmergingMarketsEquitiesMember
1685	UK Property [Member]	Member			bt:UKPropertyMember
1686	Overseas Property [Member]	Member			bt:OverseasPropertyMember
1687	Absolute Return [Member]	Member			bt:AbsoluteReturnMember
1688	Non Core Credit [Member]	Member			bt:NonCoreCreditMember
1689	Mature Infrastructure [Member]	Member			bt:MatureInfrastructureMember
1690	UK Index Linked Government Bonds [Member]	Member			bt:UKIndexLinkedGovernmentBondsMember
1691	Global Investment Grade Credit [Member]	Member			bt:GlobalInvestmentGradeCreditMember
1692	EEPS [Member]	Member			bt:EEPSMember
1693	Longevity Insurance Contract [Member]	Member			bt:LongevityInsuranceContracteMember
1694	Other Non-physical Cash [Member]	Member			bt:OtherNonPhysicalCashMember
1695	Disclosure Of Defined Benefit Plans [Line Items]	LineItems			ifrs-full:DisclosureOfDefinedBenefitPlansLineItems
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1699	101390 - Disclosure - Retirement Benefit Plan - Summary of Fair Value of Assets of BTPS Analysed by Asset Category (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlanSummaryOfFairValueOfAssetsOfBTPSAnalysedByAssetCategoryParentheticalDetail
1700	Disclosure Of Defined Benefit Plans [Table]	Table	*	*	ifrs-full:DisclosureOfDefinedBenefitPlansTable
1701	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
1702	Defined benefit plans [member]	Member			ifrs-full:DefinedBenefitPlansMember
1703	BT Pension Scheme [Member]	Member			bt:BtPensionSchemeMember
1704	Disclosure Of Defined Benefit Plans [Line Items]	LineItems			ifrs-full:DisclosureOfDefinedBenefitPlansLineItems
1705	Ordinary shares held under scheme	Concept (Monetary)	As Of	Debit	bt:OrdinarySharesHeldUnderBenefitPlan
1706	Index-linked bonds held under scheme	Concept (Monetary)	As Of	Debit	bt:IndexlinkedBondsHeldUnderBenefitPlan
1707	101400 - Disclosure - Retirement Benefit Plan - Summary of Approach Used to Set the Key IAS 19 Assumptions (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlanSummaryOfApproachUsedToSetKeyIAS19AssumptionsDetail
1708	Disclosure Of Defined Benefit Plans [Table]	Table	*	*	ifrs-full:DisclosureOfDefinedBenefitPlansTable
1709	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
1710	Defined benefit plans [member]	Member			ifrs-full:DefinedBenefitPlansMember
1711	BT Pension Scheme [Member]	Member			bt:BtPensionSchemeMember
1712	Actuarial assumptions [axis]	Axis			ifrs-full:ActuarialAssumptionsAxis
1713	Actuarial assumptions [member]	Member			ifrs-full:ActuarialAssumptionsMember
1714	Actuarial Assumption of Discount Rates [Member]	Member			ifrs-full:ActuarialAssumptionOfDiscountRatesMember
1715	Actuarial Assumption of Expected Rates Off Retail Prices Index Inflation [Member]	Member			bt:ActuarialAssumptionOfExpectedRatesOfRetailPricesIndexInflationMember
1716	Actuarial Assumption of Expected Rates of Consumer Prices Index Inflation [Member]	Member			bt:ActuarialAssumptionOfExpectedRatesOfConsumerPricesIndexInflationMember
1717	Actuarial Assumption of Expected Rates of Salary Increases [Member]	Member			ifrs-full:ActuarialAssumptionOfExpectedRatesOfSalaryIncreasesMember
1718	Actuarial Assumption of Expected Rates of Pension Increases [Member]	Member			ifrs-full:ActuarialAssumptionOfExpectedRatesOfPensionIncreasesMember
1719	Actuarial Assumption of Mortality Rates [Member]	Member			bt:ActuarialAssumptionOfMortalityRatesMember
1720	Disclosure Of Defined Benefit Plans [Line Items]	LineItems			ifrs-full:DisclosureOfDefinedBenefitPlansLineItems
1721	Approach used to set key IAS 19 assumptions	Concept (Text/String)	For Period		ifrs-full:DescriptionOfMethodsAndAssumptionsUsedInPreparingSensitivityAnalysisForActuarialAssumptions
1722	101410 - Disclosure - Retirement Benefit Plan - Summary of Approach Used to Set the Key IAS 19 Assumptions (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlanSummaryOfApproachUsedToSetKeyIAS19AssumptionsParentheticalDetail
1723	Disclosure Of Defined Benefit Plans [Table]	Table	*	*	ifrs-full:DisclosureOfDefinedBenefitPlansTable
1724	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
1725	Defined benefit plans [member]	Member			ifrs-full:DefinedBenefitPlansMember
1726	BT Pension Scheme [Member]	Member			bt:BtPensionSchemeMember
1727	Disclosure Of Defined Benefit Plans [Line Items]	LineItems			ifrs-full:DisclosureOfDefinedBenefitPlansLineItems
1728	Reduction in liabilities	Concept (Monetary)	For Period	Credit	ifrs-full:IncreaseDecreaseInNetDefinedBenefitLiabilityAsset
1729	Mortality Projections model, long-term improvement parameter, per year	Concept (Percent)	For Period		bt:MortalityProjectionsModelLongtermImprovementParameterPerYearPercentage
1730	101420 - Disclosure - Retirement Benefit Plan - Summary of Key Financial Assumptions Used to Measure Liabilities of BTPS (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlanSummaryOfKeyFinancialAssumptionsUsedToMeasureLiabilitiesOfBTPSDetail
1731	Disclosure Of Defined Benefit Plans [Table]	Table	*	*	ifrs-full:DisclosureOfDefinedBenefitPlansTable
1732	Types of interest rates [axis]	Axis			ifrs-full:TypesOfInterestRatesAxis
1733	Interest rate types [member]	Member			ifrs-full:InterestRateTypesMember
1734	Real Rates [Member]	Member			bt:RealRatesMember
1735	Nominal Rates [Member]	Member			bt:NominalRatesMember
1736	Actuarial assumptions [axis]	Axis			ifrs-full:ActuarialAssumptionsAxis
1737	Actuarial assumptions [member]	Member			ifrs-full:ActuarialAssumptionsMember
1738	Retail Price Index [Member]	Member			bt:RetailPriceIndexMember
1739	Consumer Price Index [Member]	Member			bt:ConsumerPriceIndexMember
1740	Disclosure Of Defined Benefit Plans [Line Items]	LineItems			ifrs-full:DisclosureOfDefinedBenefitPlansLineItems
1741	Rate used to discount liabilities	Concept (Percent)	As Of		ifrs-full:OtherMaterialActuarialAssumptions
1742	Inflation � increase	Concept (Percent)	As Of		ifrs-full:ActuarialAssumptionOfExpectedRatesOfInflation
1743	101430 - Disclosure - Retirement Benefit Plan - Summary of Key Financial Assumptions Used to Measure Liabilities of BTPS (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlanSummaryOfKeyFinancialAssumptionsUsedToMeasureLiabilitiesOfBTPSParentheticalDetail
1744	Implied Table	Table	*	*	implied:Table
1745	Disclosure Of Defined Benefit Plans [Abstract]	Abstract			ifrs-full:DisclosureOfDefinedBenefitPlansAbstract
1746	Maximum assumed real rate	Concept (Percent)	For Period		bt:MaximumAssumedRealRate
1747	101440 - Disclosure - Retirement Benefit Plan - Summary of Forecast Life Expectancies for BTPS Members Aged 60 (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlanSummaryOfForecastLifeExpectanciesForBTPSMembersAged60Detail
1748	Disclosure Of Defined Benefit Plans [Table]	Table	*	*	ifrs-full:DisclosureOfDefinedBenefitPlansTable
1749	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
1750	Defined benefit plans [member]	Member			ifrs-full:DefinedBenefitPlansMember
1751	Male in Lower Pay Bracket [Member]	Member			bt:MaleInLowerPayBracketMember
1752	Male in Medium Pay Bracket [Member]	Member			bt:MaleInMediumPayBracketMember
1753	Male in Higher Pay Bracket [Member]	Member			bt:MaleInHigherPayBracketMember
1754	Female in Lower Pay Bracket [Member]	Member			bt:FemaleInLowerPayBracketMember
1755	Female in Higher Pay Bracket [Member]	Member			bt:FemaleInHigherPayBracketMember
1756	Average Improvement for a Member Retiring at Age 60 in 10 Years Time [Member]	Member			bt:AverageImprovementForMemberRetiringAtAge60InTenYearsTimeMember
1757	Disclosure Of Defined Benefit Plans [Line Items]	LineItems			ifrs-full:DisclosureOfDefinedBenefitPlansLineItems
1758	Number of years	Concept (xbrli:durationItemType)	For Period		bt:LifeExpectancyInYears
1759	101450 - Disclosure - Retirement Benefit Plans - Summary of Assessed Potential Negative Impact of Key Risk (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlansSummaryOfAssessedPotentialNegativeImpactOfKeyRiskDetail
1760	Implied Table	Table	*	*	implied:Table
1761	Disclosure Of Defined Benefit Plans [Abstract]	Abstract			ifrs-full:DisclosureOfDefinedBenefitPlansAbstract
1762	Fall in discount rate	Concept (Percent)	For Period		bt:PercenageOfFallInDiscountRate
1763	Increase to inflation rate	Concept (Percent)	For Period		bt:PercentageOfIncreaseToInflationRate
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1765	101460 - Disclosure - Retirement Benefit Plan - Summary of Two Most Recent Triennial Valuations (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlanSummaryOfTwoMostRecentTriennialValuationsDetail
1766	Disclosure Of Defined Benefit Plans [Table]	Table	*	*	ifrs-full:DisclosureOfDefinedBenefitPlansTable
1767	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
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1769	BTPS [Member]	Member			bt:BTPSMember
1770	Disclosure Of Defined Benefit Plans [Line Items]	LineItems			ifrs-full:DisclosureOfDefinedBenefitPlansLineItems
1771	BTPS liabilities	Concept (Monetary)	As Of	Credit	bt:PensionLiabilities
1772	Market value of BTPS assets	Concept (Monetary)	As Of	Debit	bt:MarketValueOfPensionSchemeAssets
1773	Funding deficit	Concept (Monetary)	As Of	Credit	bt:FundingDeficit
1774	Percentage of accrued benefits covered by BTPS assets at the valuation date	Concept (Percent)	As Of		bt:PercentageOfAccruedBenefitsCoveredByPensionSchemeAssetsAtValuationDate
1775	Percentage of accrued benefits on a solvency basis covered by the BTPS assets at the valuation date	Concept (Percent)	As Of		bt:PercentageOfAccruedBenefitsOnSolvencyBasisCoveredByPensionSchemeAssetsAtValuationDate
1776	101470 - Disclosure - Retirement Benefit Plan - Summary of Prudent Long-Term Assumptions (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlanSummaryOfPrudentLongTermAssumptionsDetail
1777	Disclosure Of Defined Benefit Plans [Table]	Table	*	*	ifrs-full:DisclosureOfDefinedBenefitPlansTable
1778	Types of interest rates [axis]	Axis			ifrs-full:TypesOfInterestRatesAxis
1779	Interest rate types [member]	Member			ifrs-full:InterestRateTypesMember
1780	Nominal Rates [Member]	Member			bt:NominalRatesMember
1781	Real Rates [Member]	Member			bt:RealRatesMember
1782	Disclosure Of Defined Benefit Plans [Line Items]	LineItems			ifrs-full:DisclosureOfDefinedBenefitPlansLineItems
1783	Average single equivalent discount rate	Concept (Percent)	As Of		bt:AverageSingleEquivalentDiscountRate
1784	Average long-term increase in RPI	Concept (Percent)	As Of		bt:AverageLongtermIncreaseInRpi
1785	Average long-term increase in CPI	Concept (Percent)	As Of		bt:AverageLongtermIncreaseInCpi
1786	101480 - Disclosure - Retirement Benefit Plan - Summary of Average Life Expectancies for Members Aged 60 (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlanSummaryOfAverageLifeExpectanciesForMembersAged60Detail
1787	Disclosure Of Defined Benefit Plans [Table]	Table	*	*	ifrs-full:DisclosureOfDefinedBenefitPlansTable
1788	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
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1790	Male in Lower Pay Bracket [Member]	Member			bt:MaleInLowerPayBracketMember
1791	Male in Medium Pay Bracket [Member]	Member			bt:MaleInMediumPayBracketMember
1792	Male in Higher Pay Bracket [Member]	Member			bt:MaleInHigherPayBracketMember
1793	Female in Lower Pay Bracket [Member]	Member			bt:FemaleInLowerPayBracketMember
1794	Female in Higher Pay Bracket [Member]	Member			bt:FemaleInHigherPayBracketMember
1795	Average Improvement for a Member Retiring at Age 60 in 10 Years Time [Member]	Member			bt:AverageImprovementForMemberRetiringAtAge60InTenYearsTimeMember
1796	Disclosure Of Defined Benefit Plans [Line Items]	LineItems			ifrs-full:DisclosureOfDefinedBenefitPlansLineItems
1797	Assumptions	Concept (xbrli:durationItemType)	For Period		bt:LifeExpectancyInYears
1798	101490 - Disclosure - Retirement Benefit Plan - Summary of Payments Made to BTPS (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlanSummaryOfPaymentsMadeToBTPSDetail
1799	Disclosure Of Defined Benefit Plans [Table]	Table	*	*	ifrs-full:DisclosureOfDefinedBenefitPlansTable
1800	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
1801	Defined benefit plans [member]	Member			ifrs-full:DefinedBenefitPlansMember
1802	BTPS [Member]	Member			bt:BTPSMember
1803	Disclosure Of Defined Benefit Plans [Line Items]	LineItems			ifrs-full:DisclosureOfDefinedBenefitPlansLineItems
1804	Ordinary contributions	Concept (Monetary)	For Period	Debit	ifrs-full:ContributionsToPlanByEmployerNetDefinedBenefitLiabilityAsset
1805	Deficit contributions	Concept (Monetary)	For Period	Debit	bt:DeficitContributionsToPlanByEmployerNetDefinedBenefitLiabilityAsset
1806	Total contributions in the year	Concept (Monetary)	For Period	Debit	ifrs-full:ContributionsToPlanNetDefinedBenefitLiabilityAsset
1807	101500 - Disclosure - Retirement Benefit Plans - Schedule of Future Deficit Payments (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlansScheduleOfFutureDeficitPaymentsDetail
1808	Disclosure Of Defined Contribution Plans [Table]	Table	*	*	bt:DisclosureOfDefinedContributionPlansTable
1809	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
1810	Defined benefit plans [member]	Member			ifrs-full:DefinedBenefitPlansMember
1811	BTPS [Member]	Member			bt:BTPSMember
1812	Maturity [axis]	Axis			ifrs-full:MaturityAxis
1813	Aggregated time bands [member]	Member			ifrs-full:AggregatedTimeBandsMember
1814	2018 [Member]	Member			ifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember
1815	2019 [Member]	Member			ifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember
1816	2020 [Member]	Member			ifrs-full:LaterThanThreeYearsAndNotLaterThanFourYearsMember
1817	2021 [Member]	Member			ifrs-full:LaterThanFourYearsAndNotLaterThanFiveYearsMember
1818	2022 [Member]	Member			bt:LaterThanFiveYearsAndNotLaterThanSixYearsMember
1819	2023 [Member]	Member			bt:LaterThanSixYearsAndNotLaterThanSevenYearsMember
1820	2024 [Member]	Member			bt:LaterThanSevenYearsAndNotLaterThanEightYearsMember
1821	2025 [Member]	Member			bt:LaterThanEightYearAndNotLaterThanNineMember
1822	2026 [Member]	Member			bt:LaterThanNineYearAndNotLaterThanTenMember
1823	2027 [Member]	Member			bt:LaterThanTenYearAndNotLaterThanElevenMember
1824	2028 [Member]	Member			bt:LaterThanElevenYearAndNotLaterThanTwelveMember
1825	2029 [Member]	Member			bt:LaterThanTweleYearAndNotLaterThanThirteenMember
1826	2030 [Member]	Member			bt:LaterThanThirteenYearAndNotLaterThanFourteenMember
1827	Disclosure of deficit contribution plans [Line Items]	LineItems			bt:DisclosureOfDeficitContributionPlansLineItems
1828	Deficit contribution (�m)	Concept (Monetary)	For Period	Debit	ifrs-full:ContributionsToPlanByPlanParticipantsNetDefinedBenefitLiabilityAsset
1829	101510 - Disclosure - Retirement Benefit Plans - Schedule of Future Deficit Payments (Parenthecial) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureRetirementBenefitPlansScheduleOfFutureDeficitPaymentsParenthecialDetail
1830	Disclosure Of Defined Contribution Plans [Table]	Table	*	*	bt:DisclosureOfDefinedContributionPlansTable
1831	Defined benefit plans [axis]	Axis			ifrs-full:DefinedBenefitPlansAxis
1832	Defined benefit plans [member]	Member			ifrs-full:DefinedBenefitPlansMember
1833	BTPS [Member]	Member			bt:BTPSMember
1834	Maturity [axis]	Axis			ifrs-full:MaturityAxis
1835	Aggregated time bands [member]	Member			ifrs-full:AggregatedTimeBandsMember
1836	Payable by 30 June 2020 [Member]	Member			bt:PayableByJuneThirtyTwoThousandTwentyMember
1837	Payable by 30 June 2021 [Member]	Member			bt:PayableByJuneThirtyTwoThousandTwentyOneMember
1838	Disclosure of deficit contribution plans [Line Items]	LineItems			bt:DisclosureOfDeficitContributionPlansLineItems
1839	Contribution payable	Concept (Monetary)	For Period	Credit	bt:DeficitContributionPayable
1840	101520 - Disclosure - Own Shares - Summary of Own Shares (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureOwnSharesSummaryOfOwnSharesDetail
1841	Statement Of Changes In Equity [Table]	Table	*	*	ifrs-full:StatementOfChangesInEquityTable
1842	Components of equity [axis]	Axis			ifrs-full:ComponentsOfEquityAxis
1843	Equity [member]	Member			ifrs-full:EquityMember
1844	Treasury Shares [Member]	Member			ifrs-full:TreasurySharesMember
1845	Employee Share Ownership Trust [Member]	Member			bt:EmployeeShareOwnershipTrustMember
1846	Own Shares [Member]	Member			bt:OwnSharesMember
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1851	Executive share awards vested	Concept (Monetary)	For Period	Credit	ifrs-full:IncreaseDecreaseThroughSharebasedPaymentTransactions
1852	Ending balance	Concept (Monetary)	As Of	Credit	ifrs-full:Equity
1853	Transfer of shares to satisfy US share scheme	Concept (Monetary)	For Period	Debit	bt:TransferOfSharesToSatisfyUSShareSchemeValue
1854	Beginning balance, shares	Concept (Shares)	As Of		ifrs-full:NumberOfSharesOutstanding
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1856	Share options exercised, shares	Concept (Shares)	For Period		bt:NumberOfSharesIssuedUponStockOptionsExercised
1857	Executive share awards vested, shares	Concept (Shares)	For Period		bt:NumberOfExecutiveShareAwardsVested
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1861	101530 - Disclosure - Own Shares - Summary of Own Shares (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureOwnSharesSummaryOfOwnSharesParentheticalDetail
1862	Implied Table	Table	*	*	implied:Table
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1866	Shares held in employee share ownership trust	Concept (Shares)	As Of		bt:NumberOfSharesHeldInEmployeeShareOwnershipTrust
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1868	Repurchase of ordinary share capital	Concept (Monetary)	For Period	Credit	bt:PaymentsForRepurchaseOfOrdinaryShareCapital
1869	Proceeds from issue of own shares	Concept (Monetary)	For Period	Debit	ifrs-full:ProceedsFromSaleOrIssueOfTreasuryShares
1870	101540 - Disclosure - Share-Based Payments - Summary of Plans and Analysis of Total Charge by Type of Award (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureShareBasedPaymentsSummaryOfPlansAndAnalysisOfTotalChargeByTypeOfAwardDetail
1871	Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Table]	Table	*	*	ifrs-full:DisclosureOfTermsAndConditionsOfSharebasedPaymentArrangementTable
1872	Types of share-based payment arrangements [axis]	Axis			ifrs-full:TypesOfSharebasedPaymentArrangementsAxis
1873	Share-based payment arrangements [member]	Member			ifrs-full:SharebasedPaymentArrangementsMember
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1875	Incentive Share Plan [Member]	Member			bt:IncentiveSharePlanMember
1876	Deferred Bonus Plan [Member]	Member			bt:DeferredBonusPlanMember
1877	Retention Share Plan [Member]	Member			bt:RetentionSharePlanMember
1878	Other Plans [Member]	Member			bt:OtherPlansMember
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1880	Share/Save Plans	Concept (Monetary)	For Period	Debit	ifrs-full:ExpenseFromSharebasedPaymentTransactionsWithEmployees
1881	101550 - Disclosure - Share-Based Payments - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureShareBasedPaymentsAdditionalInformationDetail
1882	Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Table]	Table	*	*	ifrs-full:DisclosureOfTermsAndConditionsOfSharebasedPaymentArrangementTable
1883	Types of share-based payment arrangements [axis]	Axis			ifrs-full:TypesOfSharebasedPaymentArrangementsAxis
1884	Share-based payment arrangements [member]	Member			ifrs-full:SharebasedPaymentArrangementsMember
1885	Employee Saveshare Plans [Member]	Member			bt:EmployeeSavesharePlansMember
1886	Deferred Bonus Plan [Member]	Member			bt:DeferredBonusPlanMember
1887	Retention Share Plan [Member]	Member			bt:RetentionSharePlanMember
1888	Ranges of exercise prices for outstanding share options [axis]	Axis			ifrs-full:RangesOfExercisePricesForOutstandingShareOptionsAxis
1889	Ranges of exercise prices for outstanding share options [member]	Member			ifrs-full:RangesOfExercisePricesForOutstandingShareOptionsMember
1890	Five Year Plans [Member]	Member			bt:FiveYearPlansMember
1891	Three Year Plans [Member]	Member			bt:ThreeYearPlansMember
1892	Range [axis]	Axis			ifrs-full:RangeAxis
1893	Ranges [member]	Member			ifrs-full:RangesMember
1894	Bottom of Range [Member]	Member			ifrs-full:BottomOfRangeMember
1895	Top of Range [Member]	Member			ifrs-full:TopOfRangeMember
1896	Disclosure of terms and conditions of share-based payment arrangement [Line Items]	LineItems			ifrs-full:DisclosureOfTermsAndConditionsOfSharebasedPaymentArrangementLineItems
1897	Savings-related share option plan saving period basis	Concept (Text/String)	For Period		bt:SavingsrelatedshareOptionPlanSavingPeriodBasis
1898	Percentage of discount set to market price	Concept (Percent)	For Period		bt:SharebasedArrangementsPencentageOfDiscountSetToMarketPrice
1899	Share-based awards, term	Concept (Text/String)	For Period		ifrs-full:DescriptionOfMaximumTermOfOptionsGrantedForSharebasedPaymentArrangement
1900	Share-based arrangements, term (in years)	Concept (Decimal)	For Period		ifrs-full:DescriptionOfOptionLifeShareOptionsGranted
1901	Share-based awards, exercise period	Concept (xbrli:durationItemType)	For Period		bt:SharebasedArrangementsExcercisedPeriod
1902	Share-based awards, awards grant date	Concept (Text/String)	For Period		ifrs-full:DateOfGrantOfSharebasedPaymentArrangement
1903	Percentage of each award linked to total shareholder return	Concept (Percent)	For Period		bt:PercentageOfEachAwardLinkedToTotalShareholderReturn
1904	Percentage of each award linked to three-year cumulative free cash flow measure	Concept (Percent)	For Period		bt:PercentageOfEachAwardLinkedToCumulativeFreeCashFlowMeasure
1905	Percentage of growth in underlying revenue excluding transit	Concept (Percent)	For Period		bt:PercentageOfGrowthInUnderlyingRevenueExcludingTransit
1906	Cumulative period for free cash flow measure	Concept (xbrli:durationItemType)	For Period		bt:CumulativePeriodForFreeCashFlowMeasure
1907	Weighted average share price	Concept (Monetary)	For Period		ifrs-full:WeightedAverageSharePrice
1908	Options pricing model	Concept (Text/String)	For Period		ifrs-full:InformationAboutHowFairValueWasMeasuredShareOptionsGranted
1909	Options pricing model, valuation method	Concept (Text/String)	For Period		ifrs-full:DescriptionOfOptionPricingModelShareOptionsGranted
1910	Share-based awards, performance period	Concept (xbrli:durationItemType)	For Period		bt:SharebasedAwardsPerformancePeriod
1911	Share-based awards, expected life after vesting date	Concept (xbrli:durationItemType)	For Period		bt:SharebasedAwardsExpectedLifeAfterVestingDate
1912	101560 - Disclosure - Share-Based Payments - Summary of Movements in Employee Saveshare Options (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureShareBasedPaymentsSummaryOfMovementsInEmployeeSaveshareOptionsDetail
1913	Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Table]	Table	*	*	ifrs-full:DisclosureOfTermsAndConditionsOfSharebasedPaymentArrangementTable
1914	Types of share-based payment arrangements [axis]	Axis			ifrs-full:TypesOfSharebasedPaymentArrangementsAxis
1915	Share-based payment arrangements [member]	Member			ifrs-full:SharebasedPaymentArrangementsMember
1916	Employee Saveshare Plans [Member]	Member			bt:EmployeeSavesharePlansMember
1917	Disclosure of terms and conditions of share-based payment arrangement [Line Items]	LineItems			ifrs-full:DisclosureOfTermsAndConditionsOfSharebasedPaymentArrangementLineItems
1918	Outstanding at 1 April	Concept (Decimal)	As Of		ifrs-full:NumberOfOutstandingShareOptions
1919	Granted	Concept (Decimal)	For Period		ifrs-full:NumberOfShareOptionsGrantedInSharebasedPaymentArrangement
1920	Forfeited	Concept (Decimal)	For Period		ifrs-full:NumberOfShareOptionsForfeitedInSharebasedPaymentArrangement
1921	Exercised	Concept (Decimal)	For Period		ifrs-full:NumberOfShareOptionsExercisedInSharebasedPaymentArrangement
1922	Expired	Concept (Decimal)	For Period		ifrs-full:NumberOfShareOptionsExpiredInSharebasedPaymentArrangement
1923	Outstanding at 31 March	Concept (Decimal)	As Of		ifrs-full:NumberOfOutstandingShareOptions
1924	Exercisable at 31 March	Concept (Decimal)	As Of		ifrs-full:NumberOfShareOptionsExercisableInSharebasedPaymentArrangement
1925	Outstanding at 1 April	Concept (Monetary)	As Of		ifrs-full:WeightedAverageExercisePriceOfShareOptionsOutstandingInSharebasedPaymentArrangement
1926	Granted	Concept (Monetary)	For Period		ifrs-full:WeightedAverageExercisePriceOfShareOptionsGrantedInSharebasedPaymentArrangement
1927	Forfeited	Concept (Monetary)	For Period		ifrs-full:WeightedAverageExercisePriceOfShareOptionsForfeitedInSharebasedPaymentArrangement
1928	Exercised	Concept (Monetary)	For Period		ifrs-full:WeightedAverageExercisePriceOfShareOptionsExercisedInSharebasedPaymentArrangement
1929	Expired	Concept (Monetary)	For Period		ifrs-full:WeightedAverageExercisePriceOfShareOptionsExpiredInSharebasedPaymentArrangement
1930	Outstanding at 31 March	Concept (Monetary)	As Of		ifrs-full:WeightedAverageExercisePriceOfShareOptionsOutstandingInSharebasedPaymentArrangement
1931	Exercisable at 31 March	Concept (Monetary)	As Of		ifrs-full:WeightedAverageExercisePriceOfShareOptionsExercisableInSharebasedPaymentArrangement
1932	101570 - Disclosure - Share-Based Payments - Summarises Information Relating to Options Outstanding and Exercisable Under Employee Saveshare Plans (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureShareBasedPaymentsSummarisesInformationRelatingToOptionsOutstandingAndExercisableUnderEmployeeSavesharePlansDetail
1933	Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Table]	Table	*	*	ifrs-full:DisclosureOfTermsAndConditionsOfSharebasedPaymentArrangementTable
1934	Range [axis]	Axis			ifrs-full:RangeAxis
1935	Ranges [member]	Member			ifrs-full:RangesMember
1936	Bottom of Range [Member]	Member			ifrs-full:BottomOfRangeMember
1937	Top of Range [Member]	Member			ifrs-full:TopOfRangeMember
1938	Maturity [axis]	Axis			ifrs-full:MaturityAxis
1939	Aggregated time bands [member]	Member			ifrs-full:AggregatedTimeBandsMember
1940	2018 [Member]	Member			ifrs-full:NotLaterThanOneYearMember
1941	2018 [Member]	Member			ifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember
1942	2019 [Member]	Member			ifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember
1943	2020 [Member]	Member			ifrs-full:LaterThanThreeYearsAndNotLaterThanFourYearsMember
1944	2021 [Member]	Member			ifrs-full:LaterThanFourYearsAndNotLaterThanFiveYearsMember
1945	Disclosure of terms and conditions of share-based payment arrangement [Line Items]	LineItems			ifrs-full:DisclosureOfTermsAndConditionsOfSharebasedPaymentArrangementLineItems
1946	Exercise price per share	Concept (Monetary)	As Of		ifrs-full:ExercisePriceOfOutstandingShareOptions
1947	Number of outstanding options	Concept (Decimal)	As Of		ifrs-full:NumberOfOutstandingShareOptions
1948	Weighted average remaining contractual life	Concept (Decimal)	As Of		ifrs-full:WeightedAverageRemainingContractualLifeOfOutstandingShareOptions
1949	101580 - Disclosure - Share-Based Payments - Summary of Movements in Executive Share Plan Awards (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureShareBasedPaymentsSummaryOfMovementsInExecutiveSharePlanAwardsDetail
1950	Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Table]	Table	*	*	ifrs-full:DisclosureOfTermsAndConditionsOfSharebasedPaymentArrangementTable
1951	Types of share-based payment arrangements [axis]	Axis			ifrs-full:TypesOfSharebasedPaymentArrangementsAxis
1952	Share-based payment arrangements [member]	Member			ifrs-full:SharebasedPaymentArrangementsMember
1953	Incentive Share Plan [Member]	Member			bt:IncentiveSharePlanMember
1954	Deferred Bonus Plan [Member]	Member			bt:DeferredBonusPlanMember
1955	Retention Share Plan [Member]	Member			bt:RetentionSharePlanMember
1956	Disclosure of terms and conditions of share-based payment arrangement [Line Items]	LineItems			ifrs-full:DisclosureOfTermsAndConditionsOfSharebasedPaymentArrangementLineItems
1957	At 1 April 2017	Concept (Decimal)	As Of		ifrs-full:NumberOfOtherEquityInstrumentsOutstandingInSharebasedPaymentArrangement
1958	Awards granted	Concept (Decimal)	For Period		ifrs-full:NumberOfInstrumentsOtherEquityInstrumentsGranted
1959	Awards vested	Concept (Decimal)	For Period		ifrs-full:NumberOfOtherEquityInstrumentsExercisedOrVestedInSharebasedPaymentArrangement
1960	Awards lapsed	Concept (Decimal)	For Period		ifrs-full:NumberOfOtherEquityInstrumentsExpiredInSharebasedPaymentArrangement
1961	Dividend shares reinvested	Concept (Shares)	For Period		bt:NumberOfOtherEquityInstrumentsDividendSharesReinvestedInSharebasedPaymentArrangement
1962	At 31 March 2018	Concept (Decimal)	As Of		ifrs-full:NumberOfOtherEquityInstrumentsOutstandingInSharebasedPaymentArrangement
1963	101590 - Disclosure - Share-Based Payments - Summary of Fair Values and Key Assumptions Used for Valuing Grants Made Under Employee Saveshare Plans and ISP (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureShareBasedPaymentsSummaryOfFairValuesAndKeyAssumptionsUsedForValuingGrantsMadeUnderEmployeeSavesharePlansAndISPDetail
1964	Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Table]	Table	*	*	ifrs-full:DisclosureOfTermsAndConditionsOfSharebasedPaymentArrangementTable
1965	Types of share-based payment arrangements [axis]	Axis			ifrs-full:TypesOfSharebasedPaymentArrangementsAxis
1966	Share-based payment arrangements [member]	Member			ifrs-full:SharebasedPaymentArrangementsMember
1967	Employee Saveshare Plans [Member]	Member			bt:EmployeeSavesharePlansMember
1968	Incentive Share Plan [Member]	Member			bt:IncentiveSharePlanMember
1969	Range [axis]	Axis			ifrs-full:RangeAxis
1970	Ranges [member]	Member			ifrs-full:RangesMember
1971	Minimum [Member]	Member			ifrs-full:BottomOfRangeMember
1972	Top of Range [Member]	Member			ifrs-full:TopOfRangeMember
1973	Disclosure of terms and conditions of share-based payment arrangement [Line Items]	LineItems			ifrs-full:DisclosureOfTermsAndConditionsOfSharebasedPaymentArrangementLineItems
1974	Weighted average fair value	Concept (Monetary)	For Period		ifrs-full:WeightedAverageSharePriceShareOptionsGranted
1975	Weighted average share price	Concept (Monetary)	For Period		ifrs-full:WeightedAverageSharePrice
1976	Weighted average exercise price	Concept (Monetary)	For Period		ifrs-full:ExercisePriceShareOptionsGranted
1977	Expected dividend yield	Concept (Percent)	For Period		ifrs-full:ExpectedDividendAsPercentageShareOptionsGranted
1978	Risk free rates	Concept (Percent)	For Period		ifrs-full:DescriptionOfRiskFreeInterestRateShareOptionsGranted
1979	Expected volatility	Concept (Percent)	For Period		ifrs-full:DescriptionOfExpectedVolatilityShareOptionsGranted
1980	101600 - Disclosure - Investments - Summary of Investments (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureInvestmentsSummaryOfInvestmentsDetail
1981	Implied Table	Table	*	*	implied:Table
1982	Disclosure Of Investments [Abstract]	Abstract			bt:DisclosureOfInvestmentsAbstract
1983	Non-current assets	Abstract			ifrs-full:CategoriesOfNoncurrentFinancialAssetsAbstract
1984	Available-for-sale	Concept (Monetary)	As Of	Debit	ifrs-full:NoncurrentFinancialAssetsAvailableforsale
1985	Fair value through profit or loss	Concept (Monetary)	As Of	Debit	ifrs-full:NoncurrentFinancialAssetsAtFairValueThroughProfitOrLoss
1986	Non-current assets	Concept (Monetary)	As Of	Debit	ifrs-full:NoncurrentFinancialAssets
1987	Current assets	Abstract			ifrs-full:CategoriesOfCurrentFinancialAssetsAbstract
1988	Available-for-sale	Concept (Monetary)	As Of	Debit	ifrs-full:CurrentFinancialAssetsAvailableforsale
1989	Loans and receivables	Concept (Monetary)	As Of	Debit	ifrs-full:CurrentLoansAndReceivables
1990	Current financial assets	Concept (Monetary)	As Of	Debit	ifrs-full:CurrentFinancialAssets
1991	101610 - Disclosure - Investments - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureInvestmentsAdditionalInformationDetail
1992	Disclosure Of Investment Asset [Table]	Table	*	*	bt:DisclosureOfInvestmentAssetTable
1993	Currencies	Axis			bt:CurrenciesAxis
1994	Currencies	Member			bt:CurrenciesDomain
1995	Sterling [Member]	Member			currency:GBP
1996	USD [Member]	Member			currency:USD
1997	Other Currencies [Member]	Member			bt:OtherCurrenciesMember
1998	Euros [Member]	Member			currency:EUR
1999	Levels of fair value hierarchy [axis]	Axis			ifrs-full:LevelsOfFairValueHierarchyAxis
2000	All levels of fair value hierarchy [member]	Member			ifrs-full:AllLevelsOfFairValueHierarchyMember
2001	Level 2 [Member]	Member			ifrs-full:Level2OfFairValueHierarchyMember
2002	Level 3 [Member]	Member			ifrs-full:Level3OfFairValueHierarchyMember
2003	Disclosure of investment [line items]	LineItems			bt:DisclosureOfInvestmentLineItems
2004	Investments in term deposits denominated in currencies	Concept (Monetary)	As Of	Debit	bt:InvestmentsInTermDeposits
2005	Available-for-sale investments	Concept (Monetary)	As Of	Debit	ifrs-full:FinancialAssetsHeldForManagingLiquidityRisk
2006	Available-for-sale investments	Concept (Monetary)	As Of	Debit	ifrs-full:FinancialAssetsAvailableforsale
2007	101620 - Disclosure - Investments - Fair Value Hierarchy of Investments (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureInvestmentsFairValueHierarchyOfInvestmentsDetail
2008	Disclosure Of Investment Asset [Table]	Table	*	*	bt:DisclosureOfInvestmentAssetTable
2009	Levels of fair value hierarchy [axis]	Axis			ifrs-full:LevelsOfFairValueHierarchyAxis
2010	All levels of fair value hierarchy [member]	Member			ifrs-full:AllLevelsOfFairValueHierarchyMember
2011	Level 1 [Member]	Member			ifrs-full:Level1OfFairValueHierarchyMember
2012	Level 2 [Member]	Member			ifrs-full:Level2OfFairValueHierarchyMember
2013	Level 3 [Member]	Member			ifrs-full:Level3OfFairValueHierarchyMember
2014	Disclosure Of Investment Asset [Line Items]	LineItems			bt:DisclosureOfInvestmentAssetLineItems
2015	Non-current and current investments	Abstract			ifrs-full:CategoriesOfFinancialAssetsAbstract
2016	Available-for-sale investments	Concept (Monetary)	As Of	Debit	ifrs-full:FinancialAssetsAvailableforsale
2017	Fair value through profit or loss	Concept (Monetary)	As Of	Debit	ifrs-full:FinancialAssetsAtFairValueThroughProfitOrLoss
2018	Total	Concept (Monetary)	As Of	Debit	ifrs-full:FinancialAssets
2019	101630 - Disclosure - Cash and Cash Equivalents - Summary of Cash and Cash Equivalents (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureCashAndCashEquivalentsSummaryOfCashAndCashEquivalentsDetail
2020	Disclosure Of Cash And Cash Equivalents [Table]	Table	*	*	bt:DisclosureOfCashAndCashEquivalentsTable
2021	Categories of financial assets [axis]	Axis			ifrs-full:CategoriesOfFinancialAssetsAxis
2022	Financial assets, category [member]	Member			ifrs-full:FinancialAssetsCategoryMember
2023	US Deposits [Member]	Member			bt:USDepositsMember
2024	UK Deposits [Member]	Member			bt:UKDepositsMember
2025	Other Deposits [Member]	Member			bt:OtherDepositsMember
2026	Disclosure of cash and cash equivalents [line items]	LineItems			bt:DisclosureOfCashAndCashEquivalentsLineItems
2027	Cash at bank and in hand	Concept (Monetary)	As Of	Debit	ifrs-full:Cash
2028	Cash equivalents	Abstract			ifrs-full:CashEquivalentsAbstract
2029	Loans and receivables	Abstract			bt:LoansAndReceivablesAbstract
2030	Other deposits	Concept (Monetary)	As Of	Debit	ifrs-full:ShorttermDepositsClassifiedAsCashEquivalents
2031	Total cash equivalents	Concept (Monetary)	As Of	Debit	ifrs-full:CashEquivalents
2032	Total cash and cash equivalents	Concept (Monetary)	As Of	Debit	ifrs-full:CashAndCashEquivalents
2033	Bank overdrafts	Concept (Monetary)	As Of	Credit	ifrs-full:BankOverdraftsClassifiedAsCashEquivalents
2034	Cash and cash equivalents per the cash flow statement	Concept (Monetary)	As Of	Debit	ifrs-full:CashAndCashEquivalentsIfDifferentFromStatementOfFinancialPosition
2035	101640 - Disclosure - Cash and Cash Equivalents - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureCashAndCashEquivalentsAdditionalInformationDetail
2036	Cash And Cash Equivalents [Table]	Table	*	*	bt:CashAndCashEquivalentsTable
2037	Classes of assets [axis]	Axis			ifrs-full:ClassesOfAssetsAxis
2038	Assets [member]	Member			ifrs-full:ClassesOfAssetsMember
2039	Held In Countries Prevent From Accessing Cash Balance [Member]	Member			bt:HeldInCountriesPreventFromAccessingCashBalanceMember
2040	Held in Escrow Accounts or in Commercial Arrangements [Member]	Member			bt:HeldInEscrowAccountsOrInCommercialArrangementsMember
2041	Cash and Cash Equivalents [Line Items]	LineItems			bt:CashAndCashEquivalentLineItems
2042	Restricted cash	Concept (Monetary)	As Of	Debit	ifrs-full:RestrictedCashAndCashEquivalents
2043	101650 - Disclosure - Loans and Other Borrowings - Summary of Components of Capital (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureLoansAndOtherBorrowingsSummaryOfComponentsOfCapitalDetail
2044	Implied Table	Table	*	*	implied:Table
2045	Disclosure Of Detailed Information About Borrowings [Abstract]	Abstract			ifrs-full:DisclosureOfDetailedInformationAboutBorrowingsAbstract
2046	Net debt	Concept (Monetary)	As Of	Credit	bt:BorrowingsNet
2047	Total parent shareholders? equity	Concept (Monetary)	As Of	Credit	ifrs-full:EquityAttributableToOwnersOfParent
2048	Total	Concept (Monetary)	As Of	Credit	bt:Capital1
2049	101660 - Disclosure - Loans and Other Borrowings - Summary of Components of Capital (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureLoansAndOtherBorrowingsSummaryOfComponentsOfCapitalParentheticalDetail
2050	Implied Table	Table	*	*	implied:Table
2051	Disclosure Of Detailed Information About Borrowings [Abstract]	Abstract			ifrs-full:DisclosureOfDetailedInformationAboutBorrowingsAbstract
2052	Non-controlling interests	Concept (Monetary)	As Of	Credit	ifrs-full:NoncontrollingInterests
2053	101670 - Disclosure - Loans and Other Borrowings - Summary of Reconciliation From Most Directly Comparable IFRS Measure to Net Debt (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureLoansAndOtherBorrowingsSummaryOfReconciliationFromMostDirectlyComparableIFRSMeasureToNetDebtDetail
2054	Disclosure Of Detailed Information About Borrowings [Table]	Table	*	*	ifrs-full:DisclosureOfDetailedInformationAboutBorrowingsTable
2055	Borrowings by name [axis]	Axis			ifrs-full:BorrowingsByNameAxis
2056	Borrowings by name [member]	Member			ifrs-full:BorrowingsByNameMember
2057	1.625% Bond Due June 2016 [Member]	Member			bt:OnePointSixTwoFivePercentageBondDueJuneTwoThousandSixteenMember
2058	1.125% bond due March 2023 [Member]	Member			bt:OnePointOneTwoFivePercentageBondDueMarchTwoThousandTwentyThreeMember
2059	8.50% Bond Due December 2016 [Member]	Member			bt:EightPointFiveZeroPercentageBondDueDecemberTwoThousandSixteenMember
2060	3.5% Bond Due February 2017 [Member]	Member			bt:ThreePointFivePercentageBondDueFebruaryTwoThousandSeventeenMember
2061	1.25% Bond Due February 2017 [Member]	Member			bt:OnePointTwoFivePercentageBondDueFebruaryTwoThousandSeventeenMember
2062	6.625% Bond Due June 2017 [Member]	Member			bt:SixPointSixTwoFivePercentageBondDueJuneTwoThousandSeventeenMember
2063	5.95% Bond Due January 2018 [Member]	Member			bt:FivePointNineFivePercentageBondDueJanuaryTwoThousandEighteenMember
2064	3.25% Bond Due August 2018 [Member]	Member			bt:ThreePointTwoFivePercentageBondDueAugustTwoThousandEighteenMember
2065	2.35% Bond Due February 2019 [Member]	Member			bt:TwoPointThreeFivePercentageBondDueFebruaryTwoThousandNineteenMember
2066	4.38% Bond Due March 2019 [Member]	Member			bt:FourPointThreeEightPercentageBondDueMarchTwoThousandNineteenMember
2067	1.125% Bond Due June 2019 [Member]	Member			bt:OnePointOneTwoFivePercentageBondDueJuneTwoThousandNineteenMember
2068	8.625% Bond Due March 2020 [Member]	Member			bt:EightPointSixTwoFivePercentageBondDueMarchTwoThousandTwentyMember
2069	0.625% Bond Due March 2021 [Member]	Member			bt:ZeroPointSixTwoFivePercentageBondDueMarchTwoThousandTwentyOneMember
2070	0.5% Bond Due June 2022 [Member]	Member			bt:ZeroPointFivePercentageBondDueJuneTwoThousandTwentyTwoMember
2071	1% Bond Due June 2024 [Member]	Member			bt:OnePercentageBondDueJuneTwoThousandTwentyFourMember
2072	1% Bond Due November 2024 [Member]	Member			bt:OnePercentageBondDueNovemberTwoThousandTwentyFourMember
2073	3.50% Index Linked Bond Due April 2025 [Member]	Member			bt:ThreePointFiveZeroPercentageIndexLinkedBondDueAprilTwoThousandTwentyFiveMember
2074	1.5% Bond Due June 2027 [Member]	Member			bt:OnePointFivePercentageBondDueJuneTwoThousandTwentySevenMember
2075	5.75% Bond Due December 2028 [Member]	Member			bt:FivePointSevenFivePercentageBondDueDecemberTwoThousandTwentyEightMember
2076	9.125% Bond Due December 2030 [Member]	Member			bt:NinePointOneTwoFivePercentageBondDueDecemberTwoThousandThirtyMember
2077	1.75% Bond Due March 2026 [Member]	Member			bt:OnePointSevenFivePercentageBondDueMarchTwoThousandTwentySixMember
2078	6.375% Bond Due June 2037 [Member]	Member			bt:SixPointThreeSevenFivePercentageBondDueJuneTwoThousandThirtySevenMember
2079	3.125% Bond Due November 2031 [Member]	Member			bt:ThreePointOneTwoFivePercentageBondDueNovemberTwoThousandThirtyOneMember
2080	LIBOR Plus 0.95% Syndicated Loan Facilities Due April 2016 [Member]	Member			bt:LIBORPlusZeroPointNineFivePercentageSyndicatedLoanFacilitiesDueAprilTwoThousandSixteenMember
2081	3.625% bond due November 2047 [Member]	Member			bt:ThreePointSixTwoFivePercentageBondDueNovemberTwoThousandFortySevenMember
2082	2.21% Bank Loan Due December 2017 [Member]	Member			bt:TwoPointTwoOnePercentageBankLoanDueDecemberTwoThousandSeventeenMember
2083	Disclosure of detailed information about borrowings [Line Items]	LineItems			ifrs-full:DisclosureOfDetailedInformationAboutBorrowingsLineItems
2084	Loans and other borrowings	Concept (Monetary)	As Of	Credit	ifrs-full:Borrowings
2085	Cash and cash equivalents	Concept (Monetary)	As Of	Debit	ifrs-full:CashAndCashEquivalents
2086	Current asset investments	Concept (Monetary)	As Of	Debit	ifrs-full:CurrentInvestments
2087	Total	Concept (Monetary)	As Of	Credit	bt:BorrowingsNetOfCurrentAssetsAdjustments
2088	To retranslate debt balances at swap rates where hedged by currency swaps	Concept (Monetary)	As Of	Debit	bt:AdjustmentsToRetranslateDebtBalancesAtSwapRatesHedgedByCurrencySwaps
2089	To remove accrued interest applied to reflect the effective interest method and fair value adjustments	Concept (Monetary)	As Of	Debit	bt:AdjustmentsToRemoveAccruedInterestAppliedToReflectTheEffectiveInterestMethodAndFairValueAdjustments
2090	Net debt	Concept (Monetary)	As Of	Credit	bt:BorrowingsNet
2091	Total listed bonds	Concept (Monetary)	As Of	Credit	ifrs-full:BondsIssued
2092	Finance leases	Concept (Monetary)	As Of	Credit	ifrs-full:FinanceLeaseLiabilities
2093	Loan due	Concept (Monetary)	As Of	Credit	ifrs-full:LoansReceived
2094	Acquisition facility	Concept (Monetary)	As Of	Credit	bt:BorrowingsRelatedToAcquisitionFacility
2095	Other loans	Concept (Monetary)	As Of	Credit	bt:OtherLoansReceived
2096	Bank overdrafts (note 24)	Concept (Monetary)	As Of	Credit	ifrs-full:BankOverdraftsClassifiedAsCashEquivalents
2097	Total other loans and borrowings	Concept (Monetary)	As Of	Credit	ifrs-full:OtherBorrowings
2098	101680 - Disclosure - Loans and Other Borrowings - Summary of Key Components of Net Debt (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureLoansAndOtherBorrowingsSummaryOfKeyComponentsOfNetDebtParentheticalDetail
2099	Implied Table	Table	*	*	implied:Table
2100	Disclosure Of Detailed Information About Borrowings [Abstract]	Abstract			ifrs-full:DisclosureOfDetailedInformationAboutBorrowingsAbstract
2101	Increase in net debt during the year	Concept (Monetary)	As Of	Credit	bt:IncreaseInNetDebt
2102	101690 - Disclosure - Loans and Other Borrowings - Summary of Key Components of Net Debt (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureLoansAndOtherBorrowingsSummaryOfKeyComponentsOfNetDebtDetail
2103	Disclosure Of Detailed Information About Borrowings [Table]	Table	*	*	ifrs-full:DisclosureOfDetailedInformationAboutBorrowingsTable
2104	Carrying amount, accumulated depreciation, amortisation and impairment and gross carrying amount [axis]	Axis			ifrs-full:CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis
2105	Carrying amount [member]	Member			ifrs-full:CarryingAmountMember
2106	Gross Carrying Amount [Member]	Member			ifrs-full:GrossCarryingAmountMember
2107	Cash And Cash Equivalents Adjustments	Member			bt:CashAndCashEquivalentsAdjustmentsMember
2108	Current Asset Investments Adjustment	Member			bt:CurrentAssetInvestmentsAdjustmentMember
2109	Assets and liabilities [axis]	Axis			ifrs-full:AssetsAndLiabilitiesAxis
2110	Assets and liabilities [member]	Member			ifrs-full:AssetsAndLiabilitiesMember
2111	Cash Flows from Derivatives Related to Net Debt [Member]	Member			bt:CashFlowsFromDerivativesRelatedToNetDebtMember
2112	Overdrafts [Member]	Member			bt:OverdraftsMember
2113	Impact of Cross-Currency Swaps [Member]	Member			bt:ImpactOfCrossCurrencySwapsMember
2114	Removal of Accrued Interest and Fair Value Adjustments [Member]	Member			bt:RemovalOfAccruedInterestAndFairValueAdjustmentsMember
2115	Maturity [axis]	Axis			ifrs-full:MaturityAxis
2116	Aggregated time bands [member]	Member			ifrs-full:AggregatedTimeBandsMember
2117	2018 [Member]	Member			ifrs-full:NotLaterThanOneYearMember
2118	Over 12 Months [Member]	Member			ifrs-full:LaterThanOneYearMember
2119	Disclosure of detailed information about borrowings [Line Items]	LineItems			ifrs-full:DisclosureOfDetailedInformationAboutBorrowingsLineItems
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2121	Issuance/(maturities)	Concept (Monetary)	For Period	Debit	bt:IncreaseDecreaseInDebtDueToIssuanceOrMaturities
2122	Fair value movements	Concept (Monetary)	For Period	Credit	bt:IncreaseDecreaseInDebtDueToFairValueMovements
2123	Foreign exchange	Concept (Monetary)	For Period	Credit	bt:IncreaseDecreaseInDebtDueToForeignExchange
2124	Transfer to within one year	Concept (Monetary)	For Period	Credit	bt:IncreaseDecreaseInDebtDueToTransferToWithinOneYear
2125	Accrued interest movements	Concept (Monetary)	For Period	Credit	bt:IncreaseDecreaseInDebtDueToAccruedInterestMovements
2126	Debt at ending balance	Concept (Monetary)	As Of	Credit	bt:BorrowingsNet
2127	101700 - Disclosure - Loans and Other Borrowings - Summary of Reconciliation From Most Directly Comparable IFRS Measure to Net Debt (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureLoansAndOtherBorrowingsSummaryOfReconciliationFromMostDirectlyComparableIFRSMeasureToNetDebtParentheticalDetail
2128	Disclosure Of Detailed Information About Borrowings [Table]	Table	*	*	ifrs-full:DisclosureOfDetailedInformationAboutBorrowingsTable
2129	Borrowings by name [axis]	Axis			ifrs-full:BorrowingsByNameAxis
2130	Borrowings by name [member]	Member			ifrs-full:BorrowingsByNameMember
2131	1.625% Bond Due June 2016 [Member]	Member			bt:OnePointSixTwoFivePercentageBondDueJuneTwoThousandSixteenMember
2132	8.50% Bond Due December 2016 [Member]	Member			bt:EightPointFiveZeroPercentageBondDueDecemberTwoThousandSixteenMember
2133	3.5% Bond Due February 2017 [Member]	Member			bt:ThreePointFivePercentageBondDueFebruaryTwoThousandSeventeenMember
2134	1.25% Bond Due February 2017 [Member]	Member			bt:OnePointTwoFivePercentageBondDueFebruaryTwoThousandSeventeenMember
2135	6.625% Bond Due June 2017 [Member]	Member			bt:SixPointSixTwoFivePercentageBondDueJuneTwoThousandSeventeenMember
2136	5.95% Bond Due January 2018 [Member]	Member			bt:FivePointNineFivePercentageBondDueJanuaryTwoThousandEighteenMember
2137	3.25% Bond Due August 2018 [Member]	Member			bt:ThreePointTwoFivePercentageBondDueAugustTwoThousandEighteenMember
2138	2.35% Bond Due February 2019 [Member]	Member			bt:TwoPointThreeFivePercentageBondDueFebruaryTwoThousandNineteenMember
2139	4.38% Bond Due March 2019 [Member]	Member			bt:FourPointThreeEightPercentageBondDueMarchTwoThousandNineteenMember
2140	1.125% Bond Due June 2019 [Member]	Member			bt:OnePointOneTwoFivePercentageBondDueJuneTwoThousandNineteenMember
2141	8.625% Bond Due March 2020 [Member]	Member			bt:EightPointSixTwoFivePercentageBondDueMarchTwoThousandTwentyMember
2142	0.5% Bond Due June 2022 [Member]	Member			bt:ZeroPointFivePercentageBondDueJuneTwoThousandTwentyTwoMember
2143	0.625% Bond Due March 2021 [Member]	Member			bt:ZeroPointSixTwoFivePercentageBondDueMarchTwoThousandTwentyOneMember
2144	1.125% bond due March 2023 [Member]	Member			bt:OnePointOneTwoFivePercentageBondDueMarchTwoThousandTwentyThreeMember
2145	1% Bond Due June 2024 [Member]	Member			bt:OnePercentageBondDueJuneTwoThousandTwentyFourMember
2146	1% Bond Due November 2024 [Member]	Member			bt:OnePercentageBondDueNovemberTwoThousandTwentyFourMember
2147	3.50% Index Linked Bond Due April 2025 [Member]	Member			bt:ThreePointFiveZeroPercentageIndexLinkedBondDueAprilTwoThousandTwentyFiveMember
2148	1.75% Bond Due March 2026 [Member]	Member			bt:OnePointSevenFivePercentageBondDueMarchTwoThousandTwentySixMember
2149	1.5% Bond Due June 2027 [Member]	Member			bt:OnePointFivePercentageBondDueJuneTwoThousandTwentySevenMember
2150	9.125% Bond Due December 2030 [Member]	Member			bt:NinePointOneTwoFivePercentageBondDueDecemberTwoThousandThirtyMember
2151	5.75% Bond Due December 2028 [Member]	Member			bt:FivePointSevenFivePercentageBondDueDecemberTwoThousandTwentyEightMember
2152	6.375% Bond Due June 2037 [Member]	Member			bt:SixPointThreeSevenFivePercentageBondDueJuneTwoThousandThirtySevenMember
2153	LIBOR Plus 0.95% Syndicated Loan Facilities Due April 2016 [Member]	Member			bt:LIBORPlusZeroPointNineFivePercentageSyndicatedLoanFacilitiesDueAprilTwoThousandSixteenMember
2154	2.21% Bank Loan Due December 2017 [Member]	Member			bt:TwoPointTwoOnePercentageBankLoanDueDecemberTwoThousandSeventeenMember
2155	3.125% Bond Due November 2031 [Member]	Member			bt:ThreePointOneTwoFivePercentageBondDueNovemberTwoThousandThirtyOneMember
2156	3.625% bond due November 2047 [Member]	Member			bt:ThreePointSixTwoFivePercentageBondDueNovemberTwoThousandFortySevenMember
2157	Disclosure of detailed information about borrowings [Line Items]	LineItems			ifrs-full:DisclosureOfDetailedInformationAboutBorrowingsLineItems
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2161	Loans and borrowings, maturity	Concept (Text/String)	For Period		ifrs-full:BorrowingsMaturity
2162	Loans and borrowings, LIBOR interest rate	Concept (Percent)	As Of		ifrs-full:BorrowingsAdjustmentToInterestRateBasis
2163	101710 - Disclosure - Loans and Other Borrowings - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureLoansAndOtherBorrowingsAdditionalInformationDetail
2164	Disclosure Of Detailed Information About Borrowings [Table]	Table	*	*	ifrs-full:DisclosureOfDetailedInformationAboutBorrowingsTable
2165	Measurement [axis]	Axis			ifrs-full:MeasurementAxis
2166	Aggregated measurement [member]	Member			ifrs-full:AggregatedMeasurementMember
2167	At Fair Value [Member]	Member			ifrs-full:AtFairValueMember
2168	Hedged items [axis]	Axis			ifrs-full:HedgedItemsAxis
2169	Hedged items [member]	Member			ifrs-full:HedgedItemsMember
2170	Principal Repayment at Hedged Rates [Member]	Member			bt:PrincipalRepaymentAtHedgedRatesMember
2171	Disclosure of detailed information about borrowings [Line Items]	LineItems			ifrs-full:DisclosureOfDetailedInformationAboutBorrowingsLineItems
2172	Fair value of listed bonds and other long-term borrowings	Concept (Monetary)	As Of	Credit	ifrs-full:Borrowings
2173	Fair value of finance leases	Concept (Monetary)	As Of	Credit	ifrs-full:LoansReceived
2174	Principal repayments of loans and borrowings at hedged rate	Concept (Monetary)	As Of	Credit	bt:HedgedDebtOutstanding
2175	101720 - Disclosure - Loans and Other Borrowings - Summary Of Loans and Other Borrowings (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureLoansAndOtherBorrowingsSummaryOfLoansAndOtherBorrowingsDetail
2176	Implied Table	Table	*	*	implied:Table
2177	Disclosure Of Detailed Information About Borrowings [Abstract]	Abstract			ifrs-full:DisclosureOfDetailedInformationAboutBorrowingsAbstract
2178	Current liabilities	Abstract			ifrs-full:CurrentBorrowingsAndCurrentPortionOfNoncurrentBorrowingsByTypeAbstract
2179	Listed bonds	Concept (Monetary)	As Of	Credit	ifrs-full:CurrentBondsIssuedAndCurrentPortionOfNoncurrentBondsIssued
2180	Finance leases	Concept (Monetary)	As Of	Credit	ifrs-full:CurrentFinanceLeaseLiabilities
2181	Bank loans	Concept (Monetary)	As Of	Credit	ifrs-full:CurrentLoansReceivedAndCurrentPortionOfNoncurrentLoansReceived
2182	Syndicated loan facilities	Concept (Monetary)	As Of	Credit	bt:CurrentPortionOfSyndicatedLoanFacilities
2183	Other loans and bank overdrafts	Concept (Monetary)	As Of	Credit	bt:CurrentPortionOfOtherLoansAndBankOverDrafts
2184	Total current liabilities	Concept (Monetary)	As Of	Credit	ifrs-full:CurrentBorrowingsAndCurrentPortionOfNoncurrentBorrowings
2185	Non-current liabilities	Abstract			ifrs-full:NoncurrentPortionOfNoncurrentBorrowingsByTypeAbstract
2186	Listed bonds	Concept (Monetary)	As Of	Credit	ifrs-full:NoncurrentPortionOfNoncurrentBondsIssued
2187	Finance leases	Concept (Monetary)	As Of	Credit	ifrs-full:NoncurrentFinanceLeaseLiabilities
2188	Bank loans	Concept (Monetary)	As Of	Credit	ifrs-full:NoncurrentPortionOfNoncurrentLoansReceived
2189	Other loans	Concept (Monetary)	As Of	Credit	ifrs-full:NoncurrentPortionOfOtherNoncurrentBorrowings
2190	Total non-current liabilities	Concept (Monetary)	As Of	Credit	ifrs-full:LongtermBorrowings
2191	Loans and other borrowings	Concept (Monetary)	As Of	Credit	ifrs-full:Borrowings
2192	101730 - Disclosure - Loans and Other Borrowings - Summary Of Loans and Other Borrowings (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureLoansAndOtherBorrowingsSummaryOfLoansAndOtherBorrowingsParentheticalDetail
2193	Implied Table	Table	*	*	implied:Table
2194	Disclosure Of Detailed Information About Borrowings [Abstract]	Abstract			ifrs-full:DisclosureOfDetailedInformationAboutBorrowingsAbstract
2195	Collateral received on swaps	Concept (Monetary)	For Period	Credit	bt:DerivativeLiabilityCashCollateralReceived
2196	101740 - Disclosure - Loans and Other Borrowings - Summary of Principal Repayments of Loans and Other Borrowings (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureLoansAndOtherBorrowingsSummaryOfPrincipalRepaymentsOfLoansAndOtherBorrowingsDetail
2197	Disclosure Of Detailed Information About Borrowings [Table]	Table	*	*	ifrs-full:DisclosureOfDetailedInformationAboutBorrowingsTable
2198	Hedging instruments [axis]	Axis			ifrs-full:HedgingInstrumentsAxis
2199	Hedging instruments [member]	Member			ifrs-full:HedgingInstrumentsMember
2200	Carrying Amount [Member]	Member			bt:CarryingValueMember
2201	Effect of hedging and interest [Member]	Member			bt:EffectOfHedgingAndInterestMember
2202	Principal Repayment at Hedged Rates [Member]	Member			bt:PrincipalRepaymentAtHedgedRatesMember
2203	Maturity [axis]	Axis			ifrs-full:MaturityAxis
2204	Aggregated time bands [member]	Member			ifrs-full:AggregatedTimeBandsMember
2205	2018 [Member]	Member			ifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember
2206	2019 [Member]	Member			ifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember
2207	2020 [Member]	Member			ifrs-full:LaterThanThreeYearsAndNotLaterThanFourYearsMember
2208	2021 [Member]	Member			ifrs-full:LaterThanFourYearsAndNotLaterThanFiveYearsMember
2209	2023 [Member]	Member			ifrs-full:LaterThanFiveYearsMember
2210	Over 12 Months [Member]	Member			ifrs-full:LaterThanOneYearMember
2211	Disclosure of detailed information about borrowings [Line Items]	LineItems			ifrs-full:DisclosureOfDetailedInformationAboutBorrowingsLineItems
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2213	Total due for repayment after more than one year	Concept (Monetary)	As Of	Credit	ifrs-full:LongtermBorrowings
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2216	Total loans and other borrowings	Concept (Monetary)	For Period	Credit	bt:LoansAndOtherBorrowings
2217	101750 - Disclosure - Loans and Other Borrowings - Summary of Obligations Under Finance Leases (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureLoansAndOtherBorrowingsSummaryOfObligationsUnderFinanceLeasesDetail
2218	Disclosure Of Detailed Information About Borrowings [Table]	Table	*	*	ifrs-full:DisclosureOfDetailedInformationAboutBorrowingsTable
2219	Maturity [axis]	Axis			ifrs-full:MaturityAxis
2220	Aggregated time bands [member]	Member			ifrs-full:AggregatedTimeBandsMember
2221	2018 [Member]	Member			ifrs-full:NotLaterThanOneYearMember
2222	Between Two and Five Years [Member]	Member			ifrs-full:LaterThanTwoYearsAndNotLaterThanFiveYearsMember
2223	2023 [Member]	Member			ifrs-full:LaterThanFiveYearsMember
2224	Disclosure of detailed information about borrowings [Line Items]	LineItems			ifrs-full:DisclosureOfDetailedInformationAboutBorrowingsLineItems
2225	Amounts payable under finance leases	Concept (Monetary)	As Of	Credit	ifrs-full:MinimumFinanceLeasePaymentsPayable
2226	Less: future finance charges	Concept (Monetary)	As Of	Credit	ifrs-full:FutureFinanceChargeOnFinanceLease
2227	Fair value adjustments for purchase price adjustment	Concept (Monetary)	As Of	Credit	bt:FinanceLeaseFairValueAdjustmentForPriceAdjustment
2228	Total finance lease obligations	Concept (Monetary)	As Of	Credit	bt:FinanceLeaseObligations
2229	Repayment of outstanding lease obligations	Concept (Monetary)	As Of	Credit	bt:RepaymentOfOutstandingFinanceLeaseObligations
2230	Less: future finance charges	Concept (Monetary)	As Of	Credit	bt:RepaymentOfFutureFinanceChargeOnFinanceLeaseObligations
2231	Fair value adjustments for purchase price adjustment	Concept (Monetary)	As Of	Credit	bt:RepaymentOfFinanceLeaseFairValueAdjustmentForPriceAdjustment
2232	Total finance lease obligations	Concept (Monetary)	As Of	Credit	bt:RepaymentOfOutstandingFinanceLeaseObligationsNet
2233	101760 - Disclosure - Finance Expense - Summary of Finance Expense (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinanceExpenseSummaryOfFinanceExpenseDetail
2234	Disclosure Of Finance Expense And Income [Table]	Table	*	*	bt:DisclosureOfFinanceExpenseAndIncomeTable
2235	Retrospective application and retrospective restatement [axis]	Axis			ifrs-full:RetrospectiveApplicationAndRetrospectiveRestatementAxis
2236	Currently stated [member]	Member			ifrs-full:RestatedMember
2237	Before Specific Items [Member]	Member			bt:BeforeSpecificItemsMember
2238	Specific Items [Member]	Member			bt:SpecificItemsMember
2239	Disclosure Of Finance Expense And Income [Line Items]	LineItems			bt:DisclosureOfFinanceExpenseAndIncomeLineItems
2240	Interest on:	Abstract			ifrs-full:InterestCostsAbstract
2241	Financial liabilities at amortised cost and associated derivatives	Concept (Monetary)	For Period	Debit	bt:InterestExpenseOnFinancialLiabilitiesAtAmortisedCostAndAssociatedDerivatives
2242	Finance leases	Concept (Monetary)	For Period	Debit	ifrs-full:InterestExpenseOnFinanceLeases
2243	Derivatives	Concept (Monetary)	For Period	Debit	bt:InterestExpenseOnDerivatives
2244	Fair value movements on derivatives not in a designated hedge relationship	Concept (Monetary)	For Period	Credit	bt:GainsLossesOnNotDesignatedAsHedgingInstrumentFairValueHedges
2245	Reclassification of cash flow hedge from other comprehensive income	Concept (Monetary)	For Period	Debit	ifrs-full:ReclassificationAdjustmentsOnCashFlowHedgesNetOfTax
2246	Unwinding of discount on provisions	Concept (Monetary)	For Period	Debit	ifrs-full:ExpenseDueToUnwindingOfDiscountOnProvisions
2247	Total finance expense	Concept (Monetary)	For Period	Debit	ifrs-full:FinanceCosts
2248	101770 - Disclosure - Finance Expense - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinanceExpenseAdditionalInformationDetail
2249	Implied Table	Table	*	*	implied:Table
2250	Disclosure Of Finance Expense And Income [Abstract]	Abstract			bt:DisclosureOfFinanceExpenseAndIncomeAbstract
2251	Net interest cash outflow	Concept (Monetary)	For Period	Credit	ifrs-full:AdjustmentsForFinanceIncomeCost
2252	Net finance cost difference from income statement	Concept (Monetary)	For Period	Debit	bt:NetFinanceCostDifferenceFromIncomeStatement
2253	101780 - Disclosure - Finance Expense - Summary of Reconciliation of Net Finance Expense to Net Interest Cash Outflow (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinanceExpenseSummaryOfReconciliationOfNetFinanceExpenseToNetInterestCashOutflowDetail
2254	Disclosure Of Finance Expense And Income [Table]	Table	*	*	bt:DisclosureOfFinanceExpenseAndIncomeTable
2255	Retrospective application and retrospective restatement [axis]	Axis			ifrs-full:RetrospectiveApplicationAndRetrospectiveRestatementAxis
2256	Currently stated [member]	Member			ifrs-full:RestatedMember
2257	Before Specific Items [Member]	Member			bt:BeforeSpecificItemsMember
2258	Disclosure Of Finance Expense And Income [Line Items]	LineItems			bt:DisclosureOfFinanceExpenseAndIncomeLineItems
2259	Finance expense before specific items	Concept (Monetary)	For Period	Debit	ifrs-full:FinanceCosts
2260	Finance income before specific items	Concept (Monetary)	For Period	Credit	ifrs-full:FinanceIncome
2261	Total net finance expense	Concept (Monetary)	For Period	Credit	ifrs-full:FinanceIncomeCost
2262	Timing differences:	Abstract			bt:TimingDifferenceAbstract
2263	� Derivative restructuring costs	Concept (Monetary)	For Period	Debit	bt:TimingDifferencesDerivativeRestructuringCosts
2264	� Timing of coupon payments on bonds	Concept (Monetary)	For Period	Debit	bt:TimingDifferencesTimingOfCouponPaymentsOnBonds
2265	� Timing of interest receipts	Concept (Monetary)	For Period	Debit	bt:TimingDifferencesTimingOfInterestReceipts
2266	� Deferred income	Concept (Monetary)	For Period	Credit	bt:TimingDifferencesDeferredIncome
2267	Specific item � EE-related financing costs	Concept (Monetary)	For Period	Debit	bt:SpecificItemRelatedFinancingCosts
2268	Net interest cash outflow	Concept (Monetary)	For Period	Credit	ifrs-full:InterestPaidClassifiedAsOperatingActivities
2269	101790 - Disclosure - Financial Instruments and Risk Management - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinancialInstrumentsAndRiskManagementAdditionalInformationDetail
2270	Disclosure Of Nature And Extent Of Risks Arising From Financial Instruments [Table]	Table	*	*	ifrs-full:DisclosureOfNatureAndExtentOfRisksArisingFromFinancialInstrumentsTable
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2275	Aggregated time bands [member]	Member			ifrs-full:AggregatedTimeBandsMember
2276	2018 [Member]	Member			ifrs-full:NotLaterThanOneYearMember
2277	Borrowings by name [axis]	Axis			ifrs-full:BorrowingsByNameAxis
2278	Borrowings by name [member]	Member			ifrs-full:BorrowingsByNameMember
2279	Debt Maturing in September 2021 [Member]	Member			bt:DebtMaturingInSeptember2011Member
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2282	Non-Financial Assets [Member]	Member			bt:NonFinancialAssetMember
2283	Prepayments and Other Receivables [Member]	Member			bt:PrepaymentsAndOtherReceivablesMember
2284	Counterparties [axis]	Axis			ifrs-full:CounterpartiesAxis
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2286	Individually Insignificant Counterparties [Member]	Member			ifrs-full:IndividuallyInsignificantCounterpartiesMember
2287	Classes of financial assets [axis]	Axis			ifrs-full:ClassesOfFinancialAssetsAxis
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2290	Types of contracts [axis]	Axis			ifrs-full:TypesOfContractsAxis
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2293	Energy Derivative Contract [Member]	Member			bt:EnergyDerivativeContractMember
2294	Types of hedges [axis]	Axis			ifrs-full:TypesOfHedgesAxis
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2297	Disclosure of Nature and Extent of Risks Arising From Financial Instruments [Line Items]	LineItems			ifrs-full:DisclosureOfNatureAndExtentOfRisksArisingFromFinancialInstrumentsLineItems
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2299	Percentage of interest accrued	Concept (Percent)	As Of		bt:PercentageOfAdditionalInterestAccruedIfRatingDowngrade
2300	Notional amount	Concept (Monetary)	As Of		ifrs-full:NotionalAmount
2301	Expected change in financial expenses	Concept (Monetary)	For Period	Debit	bt:ExpectedIncreaseDecreaseInFinancialExpenses
2302	Term debt	Concept (Monetary)	As Of	Credit	ifrs-full:ShorttermBorrowings
2303	Undrawn committed borrowing facilities	Concept (Monetary)	As Of	Credit	ifrs-full:UndrawnBorrowingFacilities
2304	Debt maturity	Concept (Text/String)	For Period		ifrs-full:BorrowingsMaturity
2305	Trade and other receivables	Concept (Monetary)	As Of	Debit	ifrs-full:NoncurrentReceivables
2306	Trade and other receivables	Concept (Monetary)	As Of	Debit	ifrs-full:TradeAndOtherCurrentReceivables
2307	Cash collateral held	Concept (Monetary)	As Of	Debit	bt:CashCollateralHeldOnDerivativeFinancialAssets
2308	Net cash inflow from netting and credit support agreements	Concept (Monetary)	For Period	Debit	ifrs-full:OtherInflowsOutflowsOfCashClassifiedAsFinancingActivities
2309	Loss on energy contract deferred	Concept (Monetary)	For Period	Credit	ifrs-full:GainsLossesOnChangeInFairValueOfDerivatives
2310	Fair value of the energy derivative	Concept (Monetary)	As Of		ifrs-full:FinancialInstrumentsDesignatedAsHedgingInstrumentsAtFairValue
2311	101800 - Disclosure - Financial Instruments and Risk Management - Summary of Currency and Interest Rate Profile of Loans and Borrowings (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinancialInstrumentsAndRiskManagementSummaryOfCurrencyAndInterestRateProfileOfLoansAndBorrowingsDetail
2312	Disclosure Of Credit Risk Exposure [Table]	Table	*	*	ifrs-full:DisclosureOfCreditRiskExposureTable
2313	Types of interest rates [axis]	Axis			ifrs-full:TypesOfInterestRatesAxis
2314	Interest rate types [member]	Member			ifrs-full:InterestRateTypesMember
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2316	Floating Interest Rate [Member]	Member			ifrs-full:FloatingInterestRateMember
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2320	Euros [Member]	Member			currency:EUR
2321	Hedged items [axis]	Axis			ifrs-full:HedgedItemsAxis
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2324	Disclosure of credit risk exposure [Line Items]	LineItems			ifrs-full:DisclosureOfCreditRiskExposureLineItems
2325	Principal repayments of loans and borrowings at hedged rate	Concept (Monetary)	As Of	Credit	bt:HedgedDebtOutstanding
2326	Ratio of fixed to floating	Concept (Percent)	For Period		bt:RatioOfDebtToAggregateDebt
2327	fixed interest rate � Sterling	Concept (Percent)	For Period		bt:WeightedAverageEffectiveFixedInterestRate
2328	101810 - Disclosure - Financial Instruments and Risk Management - Summary of Impact on Equity, Before Tax, of a 1% Increase in Interest (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinancialInstrumentsAndRiskManagementSummaryOfImpactOnEquityBeforeTaxOf1IncreaseInInterestDetail
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2330	Sensitivity Analysis	Axis			bt:SensitivityAnalysisAxis
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2333	Sterling Strengthening [Member]	Member			bt:TenPercentageChangeInSterlingStrengtheningMember
2334	US Dollar Interest Rates [Member]	Member			bt:OnePercentageChangeInUsDollarInterestRatesMember
2335	Euro Interest Rates [Member]	Member			bt:OnePercentageChangeInEuroInterestRatesMember
2336	Disclosure Of Foreign Currency Exchange Exposure [Line Items]	LineItems			bt:DisclosureOfForeignCurrencyExchangeExposureLineItems
2337	Interest rate increase (reduce)	Concept (Monetary)	For Period	Debit	bt:IncreaseDecreaseInEquity
2338	101820 - Disclosure - Financial Instruments and Risk Management - Summary of Credit Ratings (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinancialInstrumentsAndRiskManagementSummaryOfCreditRatingsDetail
2339	Disclosure Of External Credit Exposures [Table]	Table	*	*	ifrs-full:DisclosureOfExternalCreditExposuresTable
2340	External credit grades [axis]	Axis			ifrs-full:ExternalCreditGradesAxis
2341	Entity's total for external credit grades [member]	Member			ifrs-full:EntitysTotalForExternalCreditGradesMember
2342	Moodys [Member]	Member			bt:MoodysMember
2343	Standard And Poor's [Member]	Member			bt:StandardAndPoorsMember
2344	Disclosure of external credit grades [Line Items]	LineItems			ifrs-full:DisclosureOfExternalCreditExposuresLineItems
2345	Rating	Concept (Text/String)	For Period		bt:ExternalCreditRatings
2346	Outlook	Concept (Text/String)	For Period		bt:ExternalCreditRatingsOutlook
2347	101830 - Disclosure - Financial Instruments and Risk Management - Summary of Remaining Contractually-Agreed Cash Flows Including Interest Payable for Non-Derivative Financial Liabilities on Undiscounted Basis (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinancialInstrumentsAndRiskManagementSummaryOfRemainingContractuallyAgreedCashFlowsIncludingInterestPayableForNonDerivativeFinancialLiabilitiesOnUndiscountedBasisDetail
2348	Disclosure Of Maturity Analysis For Nonderivative Financial Liabilities [Table]	Table	*	*	ifrs-full:DisclosureOfMaturityAnalysisForNonderivativeFinancialLiabilitiesTable
2349	Carrying amount, accumulated depreciation, amortisation and impairment and gross carrying amount [axis]	Axis			ifrs-full:CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis
2350	Carrying amount [member]	Member			ifrs-full:CarryingAmountMember
2351	Gross Carrying Amount [Member]	Member			ifrs-full:GrossCarryingAmountMember
2352	Interest Payments Not Yet Accrued	Member			bt:InterestPaymentsNotYetAccruedMember
2353	Fair Value Adjustments	Member			bt:FairValueAdjustmentsMember
2354	Impact Of Discounting	Member			bt:ImpactOfDiscountingMember
2355	Maturity [axis]	Axis			ifrs-full:MaturityAxis
2356	Aggregated time bands [member]	Member			ifrs-full:AggregatedTimeBandsMember
2357	2018 [Member]	Member			ifrs-full:NotLaterThanOneYearMember
2358	2018 [Member]	Member			ifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember
2359	2019 [Member]	Member			ifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember
2360	2020 [Member]	Member			ifrs-full:LaterThanThreeYearsAndNotLaterThanFourYearsMember
2361	2021 [Member]	Member			ifrs-full:LaterThanFourYearsAndNotLaterThanFiveYearsMember
2362	2023 [Member]	Member			ifrs-full:LaterThanFiveYearsMember
2363	Disclosure of maturity analysis for non-derivative financial liabilities [Line Items]	LineItems			ifrs-full:DisclosureOfMaturityAnalysisForNonderivativeFinancialLiabilitiesLineItems
2364	Loans and other borrowings �m	Concept (Monetary)	As Of	Credit	ifrs-full:BankBorrowingsUndiscountedCashFlows
2365	Interest on loans and other borrowings �m	Concept (Monetary)	As Of	Credit	ifrs-full:InterestPayable
2366	Trade and other payables �m	Concept (Monetary)	As Of	Credit	ifrs-full:TradeAndOtherPayablesUndiscountedCashFlows
2367	Provisions �m	Concept (Monetary)	As Of	Credit	bt:ProvisionsUndiscountedCashFlows
2368	Total �m	Concept (Monetary)	As Of	Credit	ifrs-full:NonderivativeFinancialLiabilitiesUndiscountedCashFlows
2369	101840 - Disclosure - Financial Instruments and Risk Management - Summary of Contractually Agreed Cash Flows in Respect of Derivative Financial Instruments (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinancialInstrumentsAndRiskManagementSummaryOfContractuallyAgreedCashFlowsInRespectOfDerivativeFinancialInstrumentsDetail
2370	Disclosure Of Maturity Analysis For Derivative Financial Liabilities [Table]	Table	*	*	ifrs-full:DisclosureOfMaturityAnalysisForDerivativeFinancialLiabilitiesTable
2371	Financial instruments measured at fair value through profit or loss because credit derivative is used to manage credit risk [axis]	Axis			ifrs-full:FinancialInstrumentsMeasuredAtFairValueThroughProfitOrLossBecauseCreditDerivativeIsUsedToManageCreditRiskAxis
2372	Financial instruments measured at fair value through profit or loss because credit derivative is used to manage credit risk [member]	Member			ifrs-full:FinancialInstrumentsMeasuredAtFairValueThroughProfitOrLossBecauseCreditDerivativeIsUsedToManageCreditRiskMember
2373	Derivatives-Analysed by Earliest Payment Date [Member]	Member			bt:DerivativesAnalysedByEarliestPaymentDateMember
2374	Derivatives-Analysed Based on Holding Instrument to Maturity [Member]	Member			bt:DerivativesAnalysedBasedOnHoldingInstrumentToMaturityMember
2375	Maturity [axis]	Axis			ifrs-full:MaturityAxis
2376	Aggregated time bands [member]	Member			ifrs-full:AggregatedTimeBandsMember
2377	2018 [Member]	Member			ifrs-full:NotLaterThanOneYearMember
2378	2019 [Member]	Member			ifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember
2379	2020 [Member]	Member			ifrs-full:LaterThanThreeYearsAndNotLaterThanFourYearsMember
2380	2021 [Member]	Member			ifrs-full:LaterThanFourYearsAndNotLaterThanFiveYearsMember
2381	2023 [Member]	Member			ifrs-full:LaterThanFiveYearsMember
2382	2018 [Member]	Member			ifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember
2383	Carrying amount, accumulated depreciation, amortisation and impairment and gross carrying amount [axis]	Axis			ifrs-full:CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis
2384	Carrying amount [member]	Member			ifrs-full:CarryingAmountMember
2385	Net Settled [Member]	Member			bt:NetSettledMember
2386	Gross Settled Inflows [Member]	Member			bt:GrossSettledInflowsMember
2387	Gross Settled Outflows [Member]	Member			bt:GrossSettledOutflowsMember
2388	Disclosure of maturity analysis for derivative financial liabilities [Line Items]	LineItems			ifrs-full:DisclosureOfMaturityAnalysisForDerivativeFinancialLiabilitiesLineItems
2389	Derivative financial liabilities net settled	Concept (Monetary)	As Of	Credit	ifrs-full:DerivativeFinancialLiabilitiesUndiscountedCashFlows
2390	101850 - Disclosure - Financial Instruments and Risk Management - Summary of Maximum Credit Risk Exposure of Financial Assets (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinancialInstrumentsAndRiskManagementSummaryOfMaximumCreditRiskExposureOfFinancialAssetsDetail
2391	Disclosure Of Credit Risk Exposure [Table]	Table	*	*	ifrs-full:DisclosureOfCreditRiskExposureTable
2392	Classes of financial instruments [axis]	Axis			ifrs-full:ClassesOfFinancialInstrumentsAxis
2393	Financial instruments, class [member]	Member			ifrs-full:ClassesOfFinancialInstrumentsMember
2394	Derivative Financial Assets [Member]	Member			bt:DerivativeFinancialAssetsMember
2395	Investments [Member]	Member			ifrs-full:EquityInvestmentsMember
2396	Trade and Other Receivable [Member]	Member			bt:TradeAndOtherReceivableMember
2397	Cash and Cash Equivalent [Member]	Member			bt:CashAndCashEquivalentMember
2398	Disclosure of credit risk exposure [Line Items]	LineItems			ifrs-full:DisclosureOfCreditRiskExposureLineItems
2399	Maximum credit risk exposure	Concept (Monetary)	As Of		ifrs-full:MaximumExposureToCreditRisk
2400	101860 - Disclosure - Financial Instruments and Risk Management - Summary of Credit Quality and Credit Concentration of Cash Equivalents, Current Asset Investments and Derivative Financial Assets (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinancialInstrumentsAndRiskManagementSummaryOfCreditQualityAndCreditConcentrationOfCashEquivalentsCurrentAssetInvestmentsAndDerivativeFinancialAssetsDetail
2401	Disclosure Of External Credit Exposures [Table]	Table	*	*	ifrs-full:DisclosureOfExternalCreditExposuresTable
2402	External credit grades [axis]	Axis			ifrs-full:ExternalCreditGradesAxis
2403	Entity's total for external credit grades [member]	Member			ifrs-full:EntitysTotalForExternalCreditGradesMember
2404	Aa2/AA and Above [Member]	Member			bt:CreditRatingOneMember
2405	Aa3/AA- [Member]	Member			bt:CreditRatingTwoMember
2406	A1/A+ [Member]	Member			bt:CreditRatingThreeMember
2407	A2/A [Member]	Member			bt:CreditRatingFourMember
2408	A3/A [Member]	Member			bt:CreditRatingFiveMember
2409	Baa1/BBB+ [Member]	Member			bt:CreditRatingSixMember
2410	Baa2/BBB and Below [Member]	Member			bt:CreditRatingSevenMember
2411	Disclosure of external credit grades [Line Items]	LineItems			ifrs-full:DisclosureOfExternalCreditExposuresLineItems
2412	Credit exposure	Concept (Monetary)	As Of		ifrs-full:CreditExposure
2413	101870 - Disclosure - Financial Instruments and Risk Management - Summary of Offsetting of Financial Assets and Liabilities (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinancialInstrumentsAndRiskManagementSummaryOfOffsettingOfFinancialAssetsAndLiabilitiesDetail
2414	Implied Table	Table	*	*	implied:Table
2415	Disclosure Of Offsetting Of Financial Assets Liabilities [Abstract]	Abstract			bt:DisclosureOfOffsettingOfFinancialAssetsLiabilitiesAbstract
2416	Amounts presented in the balance sheet, derivative financial liabilities	Concept (Monetary)	As Of	Credit	ifrs-full:GrossFinancialLiabilitiesSubjectToOffsettingEnforceableMasterNettingArrangementsOrSimilarAgreements
2417	Rights of set off with derivative counterparties, derivative financial liabilities	Concept (Monetary)	As Of	Debit	ifrs-full:FinancialInstrumentsSubjectToEnforceableMasterNettingArrangementOrSimilarAgreementNotSetOffAgainstFinancialLiabilities
2418	Cash collateral, derivative financial liabilities	Concept (Monetary)	As Of	Debit	ifrs-full:CashCollateralPledgedSubjectToEnforceableMasterNettingArrangementOrSimilarAgreementNotSetOffAgainstFinancialLiabilities
2419	Net amount, derivative financial liabilities	Concept (Monetary)	As Of	Credit	ifrs-full:NetFinancialLiabilitiesSubjectToOffsettingEnforceableMasterNettingArrangementsOrSimilarAgreements
2420	Amounts presented in the balance sheet	Concept (Monetary)	As Of	Credit	bt:GrossFinancialAssetsAndLiabilitiesSubjectToOffsettingEnforceableMasterNettingArrangementsOrSimilarAgreements
2421	Right of set off with derivative counterparties	Concept (Monetary)	As Of	Credit	bt:FinancialInstrumentsSubjectToEnforceableMasterNettingArrangementOrSimilarAgreementNotSetOffAgainstFinancialAssetsAndLiabilities
2422	Cash collateral	Concept (Monetary)	As Of	Credit	bt:CashCollateralReceivedSubjectToEnforceableMasterNettingArrangementOrSimilarAgreementNotSetOffAgainstFinancialAssetsAndLiabilities
2423	Net amount	Concept (Monetary)	As Of	Debit	bt:NetFinancialAssetsAndLiabilitiesSubjectToOffsettingEnforceableMasterNettingArrangementsOrSimilarAgreements
2424	Amounts presented in the balance sheet, derivative financial assets	Concept (Monetary)	As Of	Debit	ifrs-full:GrossFinancialAssetsSubjectToOffsettingEnforceableMasterNettingArrangementsOrSimilarAgreements
2425	Right of set off with derivative counterparties,derivative financial assets	Concept (Monetary)	As Of	Credit	ifrs-full:FinancialInstrumentsSubjectToEnforceableMasterNettingArrangementOrSimilarAgreementNotSetOffAgainstFinancialAssets
2426	Cash collateral, derivative financial assets	Concept (Monetary)	As Of	Credit	ifrs-full:CashCollateralReceivedSubjectToEnforceableMasterNettingArrangementOrSimilarAgreementNotSetOffAgainstFinancialAssets
2427	Net amount, derivative financial assets	Concept (Monetary)	As Of	Debit	ifrs-full:NetFinancialAssetsSubjectToOffsettingEnforceableMasterNettingArrangementsOrSimilarAgreements
2428	101880 - Disclosure - Financial Instruments and Risk Management - Summary of Derivative Financial Instruments are Held at Fair Value on Balance Sheet (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinancialInstrumentsAndRiskManagementSummaryOfDerivativeFinancialInstrumentsAreHeldAtFairValueOnBalanceSheetDetail
2429	Disclosure Of Derivative Financial Instruments [Table]	Table	*	*	bt:DisclosureOfDerivativeFinancialInstrumentsTable
2430	Types of hedges [axis]	Axis			ifrs-full:TypesOfHedgesAxis
2431	Hedges [member]	Member			ifrs-full:TypesOfHedgesMember
2432	Designated in a Cash Flow Hedge [Member]	Member			ifrs-full:CashFlowHedgesMember
2433	Other [Member]	Member			bt:OtherHedgesMember
2434	Disclosure Of Derivative Financial Instruments [Line Items]	LineItems			bt:DisclosureOfDerivativeFinancialInstrumentsLineItems
2435	Current asset	Concept (Monetary)	As Of	Debit	ifrs-full:CurrentDerivativeFinancialAssets
2436	Non-current asset	Concept (Monetary)	As Of	Debit	ifrs-full:NoncurrentDerivativeFinancialAssets
2437	Current liability	Concept (Monetary)	As Of	Credit	ifrs-full:CurrentDerivativeFinancialLiabilities
2438	Non-current liability	Concept (Monetary)	As Of	Credit	ifrs-full:NoncurrentDerivativeFinancialLiabilities
2439	101890 - Disclosure - Other Reserves - Summary of Other Reserves (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureOtherReservesSummaryOfOtherReservesDetail
2440	Disclosure Of Reserves Within Equity [Table]	Table	*	*	ifrs-full:DisclosureOfReservesWithinEquityTable
2441	Components of equity [axis]	Axis			ifrs-full:ComponentsOfEquityAxis
2442	Equity [member]	Member			ifrs-full:EquityMember
2443	Capital Redemption Reserve [Member]	Member			ifrs-full:CapitalRedemptionReserveMember
2444	Other Reserves [Member]	Member			ifrs-full:OtherReservesMember
2445	Equity Reserves	Axis			bt:EquityReservesAxis
2446	Equity Reserves	Member			bt:EquityReservesDomain
2447	Cash Flow Reserve [Member]	Member			ifrs-full:ReserveOfCashFlowHedgesMember
2448	Available-for-Sale Reserve [Member]	Member			ifrs-full:ReserveOfGainsAndLossesOnRemeasuringAvailableforsaleFinancialAssetsMember
2449	Translation Reserve [Member]	Member			bt:TranslationReserveMember
2450	Disclosure of reserves within equity [Line Items]	LineItems			ifrs-full:DisclosureOfReservesWithinEquityLineItems
2451	Beginning balance	Concept (Monetary)	As Of	Credit	ifrs-full:Equity
2452	Exchange differences	Concept (Monetary)	For Period	Credit	ifrs-full:OtherComprehensiveIncomeNetOfTaxExchangeDifferencesOnTranslation
2453	Net fair value gain (loss) on cash flow hedges	Concept (Monetary)	For Period	Credit	ifrs-full:OtherComprehensiveIncomeNetOfTaxCashFlowHedges
2454	Recognised in income and expense	Concept (Monetary)	For Period	Debit	ifrs-full:ReclassificationAdjustmentsOnCashFlowHedgesNetOfTax
2455	Fair value movement on available-for-sale assets	Concept (Monetary)	For Period	Credit	ifrs-full:OtherComprehensiveIncomeNetOfTaxAvailableforsaleFinancialAssets
2456	Tax recognised in other comprehensive income	Concept (Monetary)	For Period		ifrs-full:IncomeTaxRelatingToComponentsOfOtherComprehensiveIncome
2457	Transfers to realised profit	Concept (Monetary)	For Period	Debit	bt:TransfersToRealisedProfit
2458	Ending balance	Concept (Monetary)	As Of	Credit	ifrs-full:Equity
2459	101900 - Disclosure - Other Reserves - Summary of Other Reserves (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureOtherReservesSummaryOfOtherReservesParentheticalDetail
2460	Disclosure Of Reserves Within Equity [Table]	Table	*	*	ifrs-full:DisclosureOfReservesWithinEquityTable
2461	Components of equity [axis]	Axis			ifrs-full:ComponentsOfEquityAxis
2462	Equity [member]	Member			ifrs-full:EquityMember
2463	Other Reserves [Member]	Member			ifrs-full:OtherReservesMember
2464	Non-Controlling Interests [Member]	Member			ifrs-full:NoncontrollingInterestsMember
2465	Equity Reserves	Axis			bt:EquityReservesAxis
2466	Equity Reserves	Member			bt:EquityReservesDomain
2467	Cash Flow Reserve [Member]	Member			ifrs-full:ReserveOfCashFlowHedgesMember
2468	Disclosure of reserves within equity [Line Items]	LineItems			ifrs-full:DisclosureOfReservesWithinEquityLineItems
2469	Net charge to cash flow reserve relating to fair value movements on derivatives	Concept (Monetary)	As Of	Credit	bt:FairValueGainOrLossOnCashFlowHedgesThatIsReclassifiedToIncomeStatement
2470	Exchange differences in relation to retained earnings attributed to non-controlling interests	Concept (Monetary)	As Of	Credit	ifrs-full:ReserveOfExchangeDifferencesOnTranslation
2471	101910 - Disclosure - Financial Commitments and Contingent Liabilities - Disclosure of Financial Commitments (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinancialCommitmentsAndContingentLiabilitiesDisclosureOfFinancialCommitmentsDetail
2472	Implied Table	Table	*	*	implied:Table
2473	Financial Commitments [Abstract]	Abstract			bt:FinancialCommitmentsAbstract
2474	Capital commitments	Concept (Monetary)	As Of	Credit	ifrs-full:CapitalCommitments
2475	Other commitments	Concept (Monetary)	As Of	Credit	bt:OtherCommitments
2476	Device purchase commitments	Concept (Monetary)	As Of	Credit	bt:DevicePurchaseCommitments
2477	TV programme rights commitments	Concept (Monetary)	As Of	Credit	bt:TVProgrammeRightsCommitments
2478	Total	Concept (Monetary)	As Of	Credit	bt:FinancialCommitments
2479	101920 - Disclosure - Financial Commitments and Contingent Liabilities - Disclosure of Future Minimum Operating Lease Payments (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinancialCommitmentsAndContingentLiabilitiesDisclosureOfFutureMinimumOperatingLeasePaymentsDetail
2480	Disclosure Of Operating Leases Future Minimum Payments [Table]	Table	*	*	bt:DisclosureOfOperatingLeasesFutureMinimumPaymentsTable
2481	Maturity [axis]	Axis			ifrs-full:MaturityAxis
2482	Aggregated time bands [member]	Member			ifrs-full:AggregatedTimeBandsMember
2483	2018 [Member]	Member			ifrs-full:NotLaterThanOneYearMember
2484	2019 [Member]	Member			ifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember
2485	2020 [Member]	Member			ifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember
2486	2021 [Member]	Member			ifrs-full:LaterThanThreeYearsAndNotLaterThanFourYearsMember
2487	2022 [Member]	Member			ifrs-full:LaterThanFourYearsAndNotLaterThanFiveYearsMember
2488	2023 [Member]	Member			bt:LaterThanFiveYearsAndNotLaterThanSixYearsMember
2489	Thereafter [Member]	Member			bt:LaterThanSixYearsAndThereafterMember
2490	Disclosure Of Operating Leases Future Minimum Payments [Line Items]	LineItems			bt:DisclosureOfOperatingLeasesFutureMinimumPaymentsLineItems
2491	Future minimum operating lease payments	Concept (Monetary)	As Of	Credit	ifrs-full:MinimumLeasePaymentsPayableUnderNoncancellableOperatingLease
2492	101930 - Disclosure - Financial Commitments and Contingent Liabilities - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosureFinancialCommitmentsAndContingentLiabilitiesAdditionalInformationDetail
2493	Disclosure Of Financial Commitments [Table]	Table	*	*	bt:DisclosureOfFinancialCommitmentsTable
2494	Legal Entity	Axis			dei:LegalEntityAxis
2495	Entity	Member			dei:EntityDomain
2496	Hutchinson 3G Limited	Member			bt:Hutchinson3GLimitedMember
2497	Categories of related parties [axis]	Axis			ifrs-full:CategoriesOfRelatedPartiesAxis
2498	Entity's total for related parties [member]	Member			ifrs-full:EntitysTotalForRelatedPartiesMember
2499	Brazilian Tax Authority [Member]	Member			bt:BrazilianTaxAuthorityMember
2500	Temporary difference, unused tax losses and unused tax credits [axis]	Axis			ifrs-full:TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsAxis
2501	Temporary difference, unused tax losses and unused tax credits [member]	Member			ifrs-full:TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsMember
2502	One case that covers the period 2013 to 2016	Member			bt:PeriodThousandThirteenToTwoThousandSixiteenMember
2503	Eight cases took place on 21 February 2018	Member			bt:FebruaryTwentyOneTwoThousandEighteenMember
2504	Retrospective application and retrospective restatement [axis]	Axis			ifrs-full:RetrospectiveApplicationAndRetrospectiveRestatementAxis
2505	Currently stated [member]	Member			ifrs-full:RestatedMember
2506	Specific Items [Member]	Member			bt:SpecificItemsMember
2507	Disclosure of Financial Commitments [Line Items]	LineItems			bt:DisclosureOfFinancialCommitmentsLineItems
2508	Average lease term	Concept (xbrli:durationItemType)	For Period		bt:AverageLeaseTerm
2509	Average fixed rental term	Concept (xbrli:durationItemType)	For Period		bt:AverageFixedRentalTerm
2510	Losses on litigation settlements	Concept (Monetary)	For Period	Debit	ifrs-full:LossesOnLitigationSettlements
2511	value of claim	Concept (Monetary)	For Period	Credit	bt:PotentialTaxClaim
2512	Deemed consent charge, net	Concept (Monetary)	For Period	Debit	bt:DeemedConsentChargeNet
2513	101940 - Disclosure - Post Balance Sheet events - Additional Information (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosurePostBalanceSheetEventsAdditionalInformationDetail
2514	Post Balance Sheet Events [Table]	Table	*	*	bt:PostBalanceSheetEventsTable
2515	Segment consolidation items [axis]	Axis			ifrs-full:SegmentConsolidationItemsAxis
2516	Entity's total for segment consolidation items [member]	Member			ifrs-full:EntitysTotalForSegmentConsolidationItemsMember
2517	Elimination of Intersegment Amounts [Member]	Member			ifrs-full:EliminationOfIntersegmentAmountsMember
2518	Segments [axis]	Axis			ifrs-full:SegmentsAxis
2519	Segments [member]	Member			ifrs-full:SegmentsMember
2520	BT Consumer [Member]	Member			bt:BTConsumerMember
2521	EE [Member]	Member			bt:EELimitedMember
2522	Business and Public Sector [Member]	Member			bt:BusinessAndPublicSectorSegmentMember
2523	Wholesale and Ventures [Member]	Member			bt:WholesaleAndVenturesSegmentMember
2524	Non-adjusting events after reporting period [axis]	Axis			ifrs-full:NonadjustingEventsAfterReportingPeriodAxis
2525	Non-adjusting events after reporting period [member]	Member			ifrs-full:NonadjustingEventsMember
2526	Events After Reporting Period [Member]	Member			bt:EventsAfterReportingPeriodMember
2527	Announcing Or Commencing Implementation of Major Restructuring [Member]	Member			ifrs-full:AnnouncingOrCommencingImplementationOfMajorRestructuringMember
2528	Segment With Whom Transaction Is With	Axis			bt:SegmentWithWhomTransactionIsWithAxis
2529	Segment With Whom Transaction Is With	Member			bt:SegmentWithWhomTransactionIsWithDomain
2530	Consumer [Member]	Member			bt:ConsumersMember
2531	Enterprise [Member]	Member			bt:Enterprise1Member
2532	Past due status [axis]	Axis			ifrs-full:PastDueStatusAxis
2533	Past due status [member]	Member			ifrs-full:PastDueStatusMember
2534	Current [Member]	Member			ifrs-full:CurrentMember
2535	Categories of related parties [axis]	Axis			ifrs-full:CategoriesOfRelatedPartiesAxis
2536	Entity's total for related parties [member]	Member			ifrs-full:EntitysTotalForRelatedPartiesMember
2537	Ofcom [Member]	Member			bt:OfcomMember
2538	Post Balance Sheet Events [Line Items]	LineItems			bt:PostBalanceSheetEventsLineItems
2539	Internal revenue	Concept (Monetary)	For Period	Credit	ifrs-full:Revenue
2540	Operating costs	Concept (Monetary)	For Period	Debit	ifrs-full:OperatingExpense
2541	Year end	Concept (Decimal)	As Of		ifrs-full:NumberOfEmployees
2542	Average	Concept (Decimal)	For Period		ifrs-full:AverageNumberOfEmployees
2543	Trade Receivables Not Past Due	Concept (Monetary)	As Of	Debit	ifrs-full:TradeReceivables
2544	Accrued income	Concept (Monetary)	As Of	Debit	ifrs-full:AccruedIncome
2545	Frequency of spectrum secured	Concept (Decimal)	As Of		bt:FrequencyOfSpectrumSecured
2546	Total frequeny of spectrum	Concept (Decimal)	As Of		bt:FrequencyOfSpectrumNet
2547	Spectrum acquisition cost	Concept (Monetary)	For Period	Debit	bt:SpectrumAcquisitionCost
2548	Deposit with Ofcom	Concept (Monetary)	For Period	Debit	bt:DepositWithRegulator
2549	Cash in excess of deposits	Concept (Monetary)	For Period	Credit	bt:CashInExcessOfDeposits
2550	Reduction in back office and middle management roles related to restructuring programme	Concept (Monetary)	For Period	Debit	bt:ReductionInBackOfficeAndMiddleManagementRolesRelatedToRestructuringProgramme
2551	Reduction payback period	Concept (xbrli:durationItemType)	For Period		bt:ReductionPaybackPeriod
2552	Expected cash cost reduction related to restructuring programme	Concept (Monetary)	For Period	Debit	bt:ExpectedCashCostReductionRelatedToRestructuringProgramme
2553	Cost of balances of restructuring programme	Concept (Monetary)	For Period	Debit	bt:CostOfBalancesOfRestructuringProgramme
2554	Number of removal of full time equivalent roles	Concept (Integer)	For Period		bt:NumberOfRemovalOfFullTimeEquivalentRoles
2555	Further run rate synergies related to ee integration programme	Concept (Monetary)	For Period	Debit	bt:FurtherRunRateSynergiesRelatedToEEIntegrationProgramme
2556	101950 - Disclosure - Post Balance Sheet events - Summary of Segment Analysis Outlining Impacts which Change in Segment Revenue and Profit (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosurePostBalanceSheetEventsSummaryOfSegmentAnalysisOutliningImpactsWhichChangeInSegmentRevenueAndProfitDetail
2557	Post Balance Sheet Events [Table]	Table	*	*	bt:PostBalanceSheetEventsTable
2558	Segments [axis]	Axis			ifrs-full:SegmentsAxis
2559	Segments [member]	Member			ifrs-full:SegmentsMember
2560	Consumer [Member]	Member			bt:ConsumerMember
2561	Enterprise [Member]	Member			bt:EnterpriseMember
2562	Global Services [Member]	Member			bt:GlobalServicesMember
2563	Openreach [Member]	Member			bt:OpenreachMember
2564	Other [Member]	Member			ifrs-full:AllOtherSegmentsMember
2565	Segment consolidation items [axis]	Axis			ifrs-full:SegmentConsolidationItemsAxis
2566	Entity's total for segment consolidation items [member]	Member			ifrs-full:EntitysTotalForSegmentConsolidationItemsMember
2567	Segment Revenue [Member]	Member			ifrs-full:OperatingSegmentsMember
2568	Internal Revenue [Member]	Member			bt:InternalRevenueMember
2569	Retrospective application and retrospective restatement [axis]	Axis			ifrs-full:RetrospectiveApplicationAndRetrospectiveRestatementAxis
2570	Currently stated [member]	Member			ifrs-full:RestatedMember
2571	Before Specific Items [Member]	Member			bt:BeforeSpecificItemsMember
2572	Specific Items [Member]	Member			bt:SpecificItemsMember
2573	Post Balance Sheet Events [Line Items]	LineItems			bt:PostBalanceSheetEventsLineItems
2574	Revenue	Concept (Monetary)	For Period	Credit	ifrs-full:Revenue
2575	EBITDA	Concept (Monetary)	For Period	Credit	bt:EarningsBeforeInterestTaxDepreciationAmortisation
2576	Depreciation and amortisation	Concept (Monetary)	For Period	Debit	ifrs-full:DepreciationAndAmortisationExpense
2577	Operating profit (loss)	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLossFromOperatingActivities
2578	Net finance expense	Concept (Monetary)	For Period	Credit	ifrs-full:FinanceIncomeCost
2579	Share of post tax profit (loss) of associates and joint ventures	Concept (Monetary)	For Period	Credit	ifrs-full:ShareOfOtherComprehensiveIncomeOfAssociatesAndJointVenturesAccountedForUsingEquityMethodThatWillBeReclassifiedToProfitOrLossBeforeTax
2580	Profit (loss) before taxation	Concept (Monetary)	For Period	Credit	ifrs-full:ProfitLossBeforeTax
2581	101960 - Disclosure - Post Balance Sheet events - Summary of Segment Analysis Outlining Impacts which Change in Segment Revenue and Profit (Parenthetical) (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosurePostBalanceSheetEventsSummaryOfSegmentAnalysisOutliningImpactsWhichChangeInSegmentRevenueAndProfitParentheticalDetail
2582	Post Balance Sheet Events [Table]	Table	*	*	bt:PostBalanceSheetEventsTable
2583	Retrospective application and retrospective restatement [axis]	Axis			ifrs-full:RetrospectiveApplicationAndRetrospectiveRestatementAxis
2584	Currently stated [member]	Member			ifrs-full:RestatedMember
2585	Specific Items [Member]	Member			bt:SpecificItemsMember
2586	Post Balance Sheet Events [Line Items]	LineItems			bt:PostBalanceSheetEventsLineItems
2587	Specific item expense	Concept (Monetary)	For Period	Debit	bt:FinanceExpense
2588	101970 - Disclosure - Post Balance Sheet events - Summary of Segment Analysis Outlining Impacts which Change in Internal Revenue and Costs (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosurePostBalanceSheetEventsSummaryOfSegmentAnalysisOutliningImpactsWhichChangeInInternalRevenueAndCostsDetail
2589	Post Balance Sheet Events [Table]	Table	*	*	bt:PostBalanceSheetEventsTable
2590	Non-adjusting events after reporting period [axis]	Axis			ifrs-full:NonadjustingEventsAfterReportingPeriodAxis
2591	Non-adjusting events after reporting period [member]	Member			ifrs-full:NonadjustingEventsMember
2592	Post Balance Sheet Events [Member]	Member			bt:PostBalanceSheetEventsMember
2593	Enterprise [Member]	Axis			bt:SegmentWithWhomTransactionIsWithAxis
2594	Segment With Whom Transaction Is With	Member			bt:SegmentWithWhomTransactionIsWithDomain
2595	Consumer [Member]	Member			bt:ConsumersMember
2596	Enterprise [Member]	Member			bt:Enterprise1Member
2597	Global Services [Member]	Member			bt:GlobalServices1Member
2598	Openreach [Member]	Member			bt:OpenreachMember
2599	Other [Member]	Member			bt:OtherSegmentMember
2600	Consumer [Member]	Member			bt:ConsumerMember
2601	Enterprise [Member]	Member			bt:EnterpriseMember
2602	Global Services [Member]	Member			bt:GlobalServicesMember
2603	Openreach [Member]	Member			bt:Openreach1Member
2604	Segment consolidation items [axis]	Axis			ifrs-full:SegmentConsolidationItemsAxis
2605	Entity's total for segment consolidation items [member]	Member			ifrs-full:EntitysTotalForSegmentConsolidationItemsMember
2606	Elimination of Intersegment Amounts [Member]	Member			ifrs-full:EliminationOfIntersegmentAmountsMember
2607	Segments [axis]	Axis			ifrs-full:SegmentsAxis
2608	Segments [member]	Member			ifrs-full:SegmentsMember
2609	Consumer [Member]	Member			bt:ConsumerMember
2610	Enterprise [Member]	Member			bt:EnterpriseMember
2611	Global Services [Member]	Member			bt:GlobalServicesMember
2612	Openreach [Member]	Member			bt:Openreach1Member
2613	Consumer [Member]	Member			bt:ConsumersMember
2614	Enterprise [Member]	Member			bt:Enterprise1Member
2615	Global Services [Member]	Member			bt:GlobalServices1Member
2616	Openreach [Member]	Member			bt:OpenreachMember
2617	Other [Member]	Member			bt:OtherSegmentMember
2618	Post Balance Sheet Events [Line Items]	LineItems			bt:PostBalanceSheetEventsLineItems
2619	Internal revenue	Concept (Monetary)	For Period	Credit	ifrs-full:Revenue
2620	Internal cost	Concept (Monetary)	For Period	Debit	ifrs-full:OperatingExpense
2621	101980 - Disclosure - Post Balance Sheet events - Summary of Segment Analysis Outlining Impacts which Change in Capital Expenditure (Detail)	Network	*	*	http://www.btplc.com/20180331/taxonomy/role/DisclosurePostBalanceSheetEventsSummaryOfSegmentAnalysisOutliningImpactsWhichChangeInCapitalExpenditureDetail
2622	Post Balance Sheet Events [Table]	Table	*	*	bt:PostBalanceSheetEventsTable
2623	Segments [axis]	Axis			ifrs-full:SegmentsAxis
2624	Segments [member]	Member			ifrs-full:SegmentsMember
2625	Consumer [Member]	Member			bt:ConsumerMember
2626	Enterprise [Member]	Member			bt:EnterpriseMember
2627	Global Services [Member]	Member			bt:GlobalServicesMember
2628	Openreach [Member]	Member			bt:OpenreachMember
2629	Other [Member]	Member			ifrs-full:AllOtherSegmentsMember
2630	Classes of intangible assets other than goodwill [axis]	Axis			ifrs-full:ClassesOfIntangibleAssetsOtherThanGoodwillAxis
2631	Intangible assets other than goodwill [member]	Member			ifrs-full:IntangibleAssetsOtherThanGoodwillMember
2632	Intangible Assets [Member]	Member			bt:IntangibleAssetsExcludingGoodwillMember
2633	Classes of property, plant and equipment [axis]	Axis			ifrs-full:ClassesOfPropertyPlantAndEquipmentAxis
2634	Property, plant and equipment [member]	Member			ifrs-full:PropertyPlantAndEquipmentMember
2635	Property, Plant and Equipment [Member]	Member			bt:PropertyPlantAndEquipmentsMember
2636	Post Balance Sheet Events [Line Items]	LineItems			bt:PostBalanceSheetEventsLineItems
2637	Capital expenditure	Concept (Monetary)	For Period	Credit	ifrs-full:PurchaseOfPropertyPlantAndEquipmentIntangibleAssetsOtherThanGoodwillInvestmentPropertyAndOtherNoncurrentAssets