cm:Concept: us-gaap:EmployeeStockOwnershipPlanESOPTaxBenefitOfDividendsPaidToPlan
| Employee Stock Ownership Plan (ESOP), Tax Benefit of Dividends Paid to Plan |
| us-gaap:EmployeeStockOwnershipPlanESOPTaxBenefitOfDividendsPaidToPlan |
| The tax benefit resulting from dividends paid on preferred stock held by an employee stock option plan, when that stock is not a common stock equivalent. |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| For Period (duration) |
| Credit |
| false |
| us-gaap:EmployeeStockOwnershipPlanESOPTaxBenefitOfDividendsPaidToPlan |