cm:Concept: us-gaap:AvailableForSaleSecuritiesGrossRealizedGainLossNet
       
          
          | Available-for-sale Securities, Gross Realized Gain (Loss) | 
       
          
          | us-gaap:AvailableForSaleSecuritiesGrossRealizedGainLossNet | 
       
          
          | This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities. | 
       
          
          | cm:Concept | 
       
          
          | Monetary (xbrli:monetaryItemType) | 
       
          
          | For Period (duration) | 
       
          
          | Credit | 
       
          
          | false | 
       
          
          | us-gaap:AvailableForSaleSecuritiesGrossRealizedGainLossNet |