cm:Concept: us-gaap:DefinedBenefitPlanAlternativeMethodUsedToAmortizePriorServiceAmounts
| Defined Benefit Plan, Alternative Method Used to Amortize Prior Service Cost, Description |
| us-gaap:DefinedBenefitPlanAlternativeMethodUsedToAmortizePriorServiceAmounts |
| Description of alternative method used to amortize prior service cost (credit) for defined benefit plan. Includes, but is not limited to, straight-line amortization over average remaining service period of employees expected to receive benefits. |
| cm:Concept |
| Text/String (xbrli:stringItemType) |
| For Period (duration) |
| (no balance type) |
| false |
| us-gaap:DefinedBenefitPlanAlternativeMethodUsedToAmortizePriorServiceAmounts |