cm:Concept: us-gaap:DefinedBenefitPlanBenefitObligationPrescriptionDrugSubsidyReceipt
| Defined Benefit Plan, Benefit Obligation, Prescription Drug Subsidy Receipt |
| us-gaap:DefinedBenefitPlanBenefitObligationPrescriptionDrugSubsidyReceipt |
| Amount, before benefit payment, of receipt from prescription drug subsidy which increases benefit obligation of defined benefit postretirement plan. |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| For Period (duration) |
| Debit |
| false |
| us-gaap:DefinedBenefitPlanBenefitObligationPrescriptionDrugSubsidyReceipt |