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 |