cm:Concept: us-gaap:OtherComprehensiveLossHeldToMaturitySecurityAdjustmentFromAociForAccretionOfNoncreditPortionOfOttiBeforeTax
       
          
          | Other Comprehensive Loss, Held-to-maturity Security, Adjustment from AOCI for Accretion of Noncredit Portion of OTTI, before Tax | 
       
          
          | us-gaap:OtherComprehensiveLossHeldToMaturitySecurityAdjustmentFromAociForAccretionOfNoncreditPortionOfOttiBeforeTax | 
       
          
          | Amount, before tax, of adjustment from accumulated other comprehensive loss for accretion of other than temporary impairment (OTTI) on investment in held-to-maturity security. | 
       
          
          | cm:Concept | 
       
          
          | Monetary (xbrli:monetaryItemType) | 
       
          
          | For Period (duration) | 
       
          
          | Credit | 
       
          
          | false | 
       
          
          | us-gaap:OtherComprehensiveLossHeldToMaturitySecurityAdjustmentFromAociForAccretionOfNoncreditPortionOfOttiBeforeTax |