cm:Concept: us-gaap:DepositAssetsEffectOfChangeInPresentValueAssumptionsResultingInReductionsToExpectedRecoveries
| Deposit Assets, Effect of Change in Present Value Assumptions Resulting in Reductions to Expected Recoveries |
| us-gaap:DepositAssetsEffectOfChangeInPresentValueAssumptionsResultingInReductionsToExpectedRecoveries |
| Amount of increase in expense due to change in present value assumptions used to value deposit assets. |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| For Period (duration) |
| Debit |
| false |
| us-gaap:DepositAssetsEffectOfChangeInPresentValueAssumptionsResultingInReductionsToExpectedRecoveries |