cm:Concept: us-gaap:PaymentsToAcquireMortgageBackedSecuritiesMBSCategorizedAsAvailableForSale
| Payments to Acquire Mortgage-backed Securities (MBS), Available-for-sale |
| us-gaap:PaymentsToAcquireMortgageBackedSecuritiesMBSCategorizedAsAvailableForSale |
| Amount of cash outflow for purchase of 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) |
| Credit |
| false |
| us-gaap:PaymentsToAcquireMortgageBackedSecuritiesMBSCategorizedAsAvailableForSale |