cm:Concept: us-gaap:DepositAssetsEffectOfIncreaseDecreaseInInterestAccrualAssumptionOnExpectedRecoveryAmounts
| Deposit Assets, Effect of (Increase) Decrease in Interest Accrual Assumption on Expected Recovery Amounts |
| us-gaap:DepositAssetsEffectOfIncreaseDecreaseInInterestAccrualAssumptionOnExpectedRecoveryAmounts |
| Amount of (increase) decrease in expense due to change in the interest accrual assumption on deposit assets. |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| For Period (duration) |
| Credit |
| false |
| us-gaap:DepositAssetsEffectOfIncreaseDecreaseInInterestAccrualAssumptionOnExpectedRecoveryAmounts |