cm:Concept: us-gaap:ShortDurationInsuranceContractDiscountedLiabilityDiscountNetOfReinsurance
| Short-duration Insurance Contract, Discounted Liability, Discount, Net of Reinsurance |
| us-gaap:ShortDurationInsuranceContractDiscountedLiabilityDiscountNetOfReinsurance |
| Amount of cumulative discount, after effect of reinsurance, related to time value of money deducted from short-duration insurance contract liability reported at present value. |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| As Of (instant) |
| Debit |
| false |
| us-gaap:ShortDurationInsuranceContractDiscountedLiabilityDiscountNetOfReinsurance |