cm:Concept: us-gaap:DebtSecuritiesHeldToMaturityAllowanceForCreditLossWriteoff
| Debt Securities, Held-to-maturity, Allowance for Credit Loss, Writeoff |
| us-gaap:DebtSecuritiesHeldToMaturityAllowanceForCreditLossWriteoff |
| Amount of writeoff of investment in debt security, measured at amortized cost (held-to-maturity), charged against allowance for credit loss. |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| For Period (duration) |
| Debit |
| false |
| us-gaap:DebtSecuritiesHeldToMaturityAllowanceForCreditLossWriteoff |