Segment Information (Details) - Prior Year Development and Catastrophes - Reportable Segments [Member] - USD ($) $ in Millions |
3 Months Ended | |
|---|---|---|
Mar. 31, 2026 |
Mar. 31, 2025 |
|
| Segment reporting information | ||
| Net favorable prior year reserve development | $ 413 | $ 378 |
| Catastrophe losses | 761 | 2,266 |
| Business Insurance [Member] | ||
| Segment reporting information | ||
| Net favorable prior year reserve development | 162 | 74 |
| Catastrophe losses | 379 | 509 |
| Bond & Specialty Insurance [Member] | ||
| Segment reporting information | ||
| Net favorable prior year reserve development | 65 | 67 |
| Catastrophe losses | 8 | 19 |
| Personal Insurance [Member] | ||
| Segment reporting information | ||
| Net favorable prior year reserve development | 186 | 237 |
| Catastrophe losses | $ 374 | $ 1,738 |