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Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($) $ in Millions | 9 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net of loss reserves discount | $ 42 | $ 37
Increase (decrease) in estimates for claims | 19 | (12)
Adjustment expense | 2 | 3
Insurance |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Unfavorable reserve development net of premium offsets | 32 | 2
Reinsurance & Monoline Excess |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Unfavorable reserve development net of premium offsets |  | $ 5
Favorable reserve development net of premium offsets | $ 34 |

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