Segment Information (Details) - Prior Year Development and Catastrophes - Reportable Segments [Member] - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2025 |
Jun. 30, 2024 |
Jun. 30, 2025 |
Jun. 30, 2024 |
|
| Segment reporting information | ||||
| Net prior year reserve development | $ 315 | $ 230 | $ 693 | $ 321 |
| Catastrophe losses | 927 | 1,509 | 3,193 | 2,221 |
| Business Insurance [Member] | ||||
| Segment reporting information | ||||
| Net prior year reserve development | 79 | 34 | 153 | 34 |
| Catastrophe losses | 368 | 389 | 877 | 598 |
| Bond & Specialty Insurance [Member] | ||||
| Segment reporting information | ||||
| Net prior year reserve development | 81 | 24 | 148 | 48 |
| Catastrophe losses | 5 | 40 | 24 | 45 |
| Personal Insurance [Member] | ||||
| Segment reporting information | ||||
| Net prior year reserve development | 155 | 172 | 392 | 239 |
| Catastrophe losses | $ 554 | $ 1,080 | $ 2,292 | $ 1,578 |