cm:Concept: us-gaap:IncreaseDecreaseInPneumoconiosisBenefitObligationsNoncurrent
       
          
          | Increase (Decrease) in Pneumoconiosis Benefit Obligations Noncurrent | 
       
       
          
          | us-gaap:IncreaseDecreaseInPneumoconiosisBenefitObligationsNoncurrent | 
       
       
          
          | The increase (decrease) during the reporting period in the amount due to fund payment obligations to former employees suffering from pneumoconiosis. | 
       
       
          
          | cm:Concept | 
       
       
          
          | Monetary (xbrli:monetaryItemType) | 
       
       
          
          | For Period (duration) | 
       
       
          
          | Debit | 
       
       
          
          | false | 
       
       
          
          | us-gaap:IncreaseDecreaseInPneumoconiosisBenefitObligationsNoncurrent |