cm:Concept: us-gaap:DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1
| Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement |
| us-gaap:DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1 |
| Amount of gain (loss) recognized in net periodic benefit (cost) credit from irrevocable action relieving primary responsibility for benefit obligation and eliminating risk related to obligation and assets used to effect settlement. |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| For Period (duration) |
| Credit |
| false |
| us-gaap:DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1 |