Segment Information (Details) - Assets by Segment - USD ($) $ in Millions |
Mar. 31, 2020 |
Dec. 31, 2019 |
|---|---|---|
| Assets by segment | ||
| Total assets | $ 109,436 | $ 110,122 |
| Reportable Segments [Member] | ||
| Assets by segment | ||
| Total assets | 108,881 | 109,510 |
| Reportable Segments [Member] | Business Insurance [Member] | ||
| Assets by segment | ||
| Total assets | 83,525 | 83,896 |
| Reportable Segments [Member] | Bond & Specialty Insurance [Member] | ||
| Assets by segment | ||
| Total assets | 8,690 | 8,599 |
| Reportable Segments [Member] | Personal Insurance [Member] | ||
| Assets by segment | ||
| Total assets | 16,666 | 17,015 |
| Other assets [Member] | ||
| Assets by segment | ||
| Total assets | $ 555 | $ 612 |