cm:Concept: us-gaap:FinancialGuaranteeInsuranceContractsAcceleratedPremiumRevenueAmount
| Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Amount |
| us-gaap:FinancialGuaranteeInsuranceContractsAcceleratedPremiumRevenueAmount |
| This element represents the amount of premium revenue on financial guarantee insurance contracts recognized during the period that was accelerated for revenue recognition purposes. |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| For Period (duration) |
| Credit |
| false |
| us-gaap:FinancialGuaranteeInsuranceContractsAcceleratedPremiumRevenueAmount |