cm:Concept: us-gaap:DebtSecuritiesAvailableForSaleAllowanceForCreditLossNotPreviouslyRecorded
| Debt Securities, Available-for-sale, Allowance for Credit Loss, Not Previously Recorded |
| us-gaap:DebtSecuritiesAvailableForSaleAllowanceForCreditLossNotPreviouslyRecorded |
| Amount of expense 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), for which no credit loss was previously recorded. |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| For Period (duration) |
| Debit |
| false |
| us-gaap:DebtSecuritiesAvailableForSaleAllowanceForCreditLossNotPreviouslyRecorded |