cm:Concept: us-gaap:DebtSecuritiesAvailableForSaleChangeInPresentValueInterestIncome
| Debt Securities, Available-for-sale, Change in Present Value, Interest Income |
| us-gaap:DebtSecuritiesAvailableForSaleChangeInPresentValueInterestIncome |
| Amount of interest income on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), from change in present value of expected cash flows, attributable to passage of time. |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| For Period (duration) |
| Credit |
| false |
| us-gaap:DebtSecuritiesAvailableForSaleChangeInPresentValueInterestIncome |