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Reinsurance and Indemnification - Canada Catastrophe Excess of Loss Reinsurance Contract (Narrative) (Details) $ in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2025 USD ($) reinstatement | Dec. 31, 2025 CAD ($) reinstatement | Dec. 31, 2024 USD ($) | Dec. 31, 2023 USD ($)
Reinsurance Retention Policy [Line Items] |  |  |  |
Property-liability insurance premiums earned | $ 2,681 |  | $ 2,260 | $ 2,036
Catastrophe Reinsurance Program |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Property-liability insurance premiums earned | $ 1,230 |  | $ 1,110 | $ 1,020
Canada Catastrophe Excess of Loss | Catastrophe Reinsurance Program |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Limit of aggregate losses under aggregate excess agreement |  | $ 478 |  |
Retention amount for catastrophe reinsurance agreement contracts |  | $ 100 |  |
Number of reinstatements each year for each contract | reinstatement | 1 | 1 |  |

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