cm:Concept: us-gaap:DebtSecuritiesAvailableForSaleAllowanceForCreditLossSellBeforeRecovery
| Debt Securities, Available-for-sale, Allowance for Credit Loss, Sell before Recovery |
| us-gaap:DebtSecuritiesAvailableForSaleAllowanceForCreditLossSellBeforeRecovery |
| Amount of decrease in allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) from intent to sell or more likely than not will be required to sell before recovery of amortized cost basis. |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| For Period (duration) |
| Debit |
| false |
| us-gaap:DebtSecuritiesAvailableForSaleAllowanceForCreditLossSellBeforeRecovery |