cm:Concept: us-gaap:ProceedsFromSaleOfMortgageBackedSecuritiesMBSCategorizedAsAvailableForSale
| Proceeds from Sale of Mortgage-backed Securities (MBS), Available-for-sale |
| us-gaap:ProceedsFromSaleOfMortgageBackedSecuritiesMBSCategorizedAsAvailableForSale |
| Amount of cash inflow from sale of investment in mortgage-backed security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| For Period (duration) |
| Debit |
| false |
| us-gaap:ProceedsFromSaleOfMortgageBackedSecuritiesMBSCategorizedAsAvailableForSale |