cm:Concept: us-gaap:DebtSecuritiesAvailableForSalePurchasedWithCreditDeteriorationDiscountPremium
| Debt Securities, Available-for-sale, Purchased with Credit Deterioration, Discount (Premium) |
| us-gaap:DebtSecuritiesAvailableForSalePurchasedWithCreditDeteriorationDiscountPremium |
| Amount of discount (premium) attributable to other factors on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), purchased with credit deterioration. |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| For Period (duration) |
| Credit |
| false |
| us-gaap:DebtSecuritiesAvailableForSalePurchasedWithCreditDeteriorationDiscountPremium |