cm:Concept: us-gaap:DefinedBenefitPlanAccountingPolicyElectionMeasurementDate
| Defined Benefit Plan, Plan Assets, Accounting Policy Election, Measurement Date |
| us-gaap:DefinedBenefitPlanAccountingPolicyElectionMeasurementDate |
| Month-end measurement date applicable when an accounting policy election is made to measure plan assets and benefit obligations as of the month-end that is closest to the fiscal year-end, in CCYY-MM-DD format. |
| cm:Concept |
| Date (xbrli:dateItemType) |
| For Period (duration) |
| (no balance type) |
| false |
| us-gaap:DefinedBenefitPlanAccountingPolicyElectionMeasurementDate |