cm:Concept: us-gaap:ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital
       
          
          | Proceeds from Equity Method Investment, Distribution, Return of Capital | 
       
       
          
          | us-gaap:ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital | 
       
       
          
          | Amount of distribution received from equity method investee for return of investment, classified as investing activities. Excludes distribution for return on investment, classified as operating activities. | 
       
       
          
          | cm:Concept | 
       
       
          
          | Monetary (xbrli:monetaryItemType) | 
       
       
          
          | For Period (duration) | 
       
       
          
          | Debit | 
       
       
          
          | false | 
       
       
          
          | us-gaap:ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital |