cm:Concept: us-gaap:ForeignReinsuranceTransactionsPremiumAdditionsToUnderwritingBalances
| Foreign Reinsurance Transactions, Premium Additions to Underwriting Balances |
| us-gaap:ForeignReinsuranceTransactionsPremiumAdditionsToUnderwritingBalances |
| The amount recorded as additions to net premiums written minus net losses paid minus the sum of loss adjustment expenses and underwriting costs and expenses on insurance ceded to companies outside the United States. |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| For Period (duration) |
| Credit |
| false |
| us-gaap:ForeignReinsuranceTransactionsPremiumAdditionsToUnderwritingBalances |