cm:Concept: us-gaap:DefinedBenefitPlanDescriptionOfDirectionAndPatternOfChangeForAssumedHealthCareCostTrendRate
| Defined Benefit Plan, Assumed Health Care Cost Trend Rate, Description |
| us-gaap:DefinedBenefitPlanDescriptionOfDirectionAndPatternOfChangeForAssumedHealthCareCostTrendRate |
| Description of direction and pattern of change in assumed health care cost trend rate used to measure expected cost of benefits for defined benefit postretirement plan. |
| cm:Concept |
| Text/String (xbrli:stringItemType) |
| For Period (duration) |
| (no balance type) |
| false |
| us-gaap:DefinedBenefitPlanDescriptionOfDirectionAndPatternOfChangeForAssumedHealthCareCostTrendRate |