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Segment Information (Details) - Prior Year Development and Catastrophes - Reportable Segments [Member] - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Segment reporting information |  |  |  |
Net favorable (unfavorable) prior year reserve development | $ 22 | $ 126 | $ 715 | $ 447
Catastrophe losses | 402 | 939 | 3,595 | 3,160
Business Insurance [Member] |  |  |  |
Segment reporting information |  |  |  |
Net favorable (unfavorable) prior year reserve development | (125) | (91) | 28 | (57)
Catastrophe losses | 139 | 340 | 1,016 | 938
Bond & Specialty Insurance [Member] |  |  |  |
Segment reporting information |  |  |  |
Net favorable (unfavorable) prior year reserve development | 43 | 36 | 191 | 84
Catastrophe losses | 0 | 4 | 24 | 49
Personal Insurance [Member] |  |  |  |
Segment reporting information |  |  |  |
Net favorable (unfavorable) prior year reserve development | 104 | 181 | 496 | 420
Catastrophe losses | $ 263 | $ 595 | $ 2,555 | $ 2,173

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- Definition Catastrophe losses included in claims and claim adjustment expenses + References No definition available. + Details
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