cm:Concept: us-gaap:PaymentsToAcquireAvailableForSaleSecurities
       
          
          | Payments to Acquire Available-for-sale Securities | 
       
       
          
          | us-gaap:PaymentsToAcquireAvailableForSaleSecurities | 
       
       
          
          | The cash outflow to acquire debt and equity securities not classified as either held-to-maturity securities or trading securities which would be classified as available-for-sale securities and reported at fair value, with unrealized gains and losses excluded from earnings and reported in a separate component of shareholders' equity. | 
       
       
          
          | cm:Concept | 
       
       
          
          | Monetary (xbrli:monetaryItemType) | 
       
       
          
          | For Period (duration) | 
       
       
          
          | Credit | 
       
       
          
          | false | 
       
       
          
          | us-gaap:PaymentsToAcquireAvailableForSaleSecurities |