cm:Concept: us-gaap:CertainLoansAcquiredInTransferAccountedForAsHeldtomaturityDebtSecuritiesOtherthanTemporaryImpairmentChargesCreditLoss
| Certain Loans Acquired in Transfer Accounted for as Held-to-maturity Debt Securities, Other-than-Temporary Impairment Charges, Credit Loss |
| us-gaap:CertainLoansAcquiredInTransferAccountedForAsHeldtomaturityDebtSecuritiesOtherthanTemporaryImpairmentChargesCreditLoss |
| Amount of expense to write-down the amortized cost basis of loans, after acquisition, to fair value when the decline in the fair value is determined to be other than temporary (OTTI) for loans accounted for as held-to-maturity debt securities. |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| For Period (duration) |
| Debit |
| false |
| us-gaap:CertainLoansAcquiredInTransferAccountedForAsHeldtomaturityDebtSecuritiesOtherthanTemporaryImpairmentChargesCreditLoss |