cm:Concept: us-gaap:OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesBeforeTax
       
          
          | Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, before Tax | 
       
          
          | us-gaap:OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesBeforeTax | 
       
          
          | Amount before tax and reclassification adjustments of unrealized holding gain (loss) related to transfers of securities into the available-for-sale classification and out of the held-to-maturity classification. | 
       
          
          | cm:Concept | 
       
          
          | Monetary (xbrli:monetaryItemType) | 
       
          
          | For Period (duration) | 
       
          
          | Credit | 
       
          
          | false | 
       
          
          | us-gaap:OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesBeforeTax |