cm:Concept: us-gaap:ShortdurationInsuranceContractsHistoricalClaimsDurationYearThree
| Short-duration Insurance Contracts, Historical Claims Duration, Year Three |
| us-gaap:ShortdurationInsuranceContractsHistoricalClaimsDurationYearThree |
| Percentage of average annual payout, after reinsurance, in the third year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts. |
| cm:Concept |
| Percent (num:percentItemType) |
| As Of (instant) |
| (no balance type) |
| false |
| us-gaap:ShortdurationInsuranceContractsHistoricalClaimsDurationYearThree |