cm:Concept: us-gaap:DebtSecuritiesHeldToMaturityChangeInPresentValueInterestIncome
| Debt Securities, Held-to-maturity, Change in Present Value, Interest Income |
| us-gaap:DebtSecuritiesHeldToMaturityChangeInPresentValueInterestIncome |
| Amount of interest income on investment in debt security measured at amortized cost (held-to-maturity), 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:DebtSecuritiesHeldToMaturityChangeInPresentValueInterestIncome |