cm:Concept: us-gaap:DefinedBenefitPlanPlanAssetsLevel3ReconciliationDecreaseForSettlement
| Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Decrease for Settlement |
| us-gaap:DefinedBenefitPlanPlanAssetsLevel3ReconciliationDecreaseForSettlement |
| Amount, measured using unobservable input, of decrease in plan asset of defined benefit plan from settlement of trade associated with underlying investment. |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| For Period (duration) |
| Credit |
| false |
| us-gaap:DefinedBenefitPlanPlanAssetsLevel3ReconciliationDecreaseForSettlement |