cm:Concept: us-gaap:EmployeeStockOwnershipPlanESOPGainLossOnTransactionsInDeferredShares
| Employee Stock Ownership Plan (ESOP), Gain (Loss) on Transactions in Deferred Shares |
| us-gaap:EmployeeStockOwnershipPlanESOPGainLossOnTransactionsInDeferredShares |
| The difference between cost and fair value added or subtracted from paid-in capital, arising from contribution of shares to an ESOP or commitments-to-release ESOP shares. |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| For Period (duration) |
| Credit |
| false |
| us-gaap:EmployeeStockOwnershipPlanESOPGainLossOnTransactionsInDeferredShares |