cm:Concept: us-gaap:DefinedBenefitPlanPlanAssetsLevel3ReconciliationIncreaseForPurchase
| Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase for Purchase |
| us-gaap:DefinedBenefitPlanPlanAssetsLevel3ReconciliationIncreaseForPurchase |
| Amount, measured using unobservable input, of increase in plan asset of defined benefit plan from purchase. |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| For Period (duration) |
| Debit |
| false |
| us-gaap:DefinedBenefitPlanPlanAssetsLevel3ReconciliationIncreaseForPurchase |