cm:Concept: us-gaap:DefinedBenefitPlanActualReturnOnPlanAssets
       
          
          | Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss) | 
       
          
          | us-gaap:DefinedBenefitPlanActualReturnOnPlanAssets | 
       
          
          | Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses. | 
       
          
          | cm:Concept | 
       
          
          | Monetary (xbrli:monetaryItemType) | 
       
          
          | For Period (duration) | 
       
          
          | Debit | 
       
          
          | false | 
       
          
          | us-gaap:DefinedBenefitPlanActualReturnOnPlanAssets |