| **Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions** | 12 Months Ended |
|----------------------------------------------------------------------------------------|-----------------|
||
| Total revenues                                                                         | $ 11,908        | $ 10,808 | $ 10,767 |
| Total operating revenues                                                               | 11,788          | 10,862 | 10,738 |
| Net investment gains (losses)                                                          | 120             | (54) | 29 |
| Operating Segments | Specialty                                                         |            |   |   |
| Total revenues                                                                         | 5,004           | 4,585 | 4,491 |
| Operating Segments | Commercial                                                        |            |   |   |
| Total revenues                                                                         | 4,199           | 3,861 | 3,796 |
| Operating Segments | International                                                     |            |   |   |
| Total revenues                                                                         | 1,114           | 998 | 1,037 |
| Operating Segments | Life & Group                                                  |            |   |   |
| Total revenues                                                                         | 1,457           | 1,355 | 1,340 |
| Operating Segments | Corporate & Other                                             |            |   |   |
| Total revenues                                                                         | 21              | 69 | 81 |
| Eliminations                                                                           |            |   |   |
| Total revenues                                                                         | (7)             | (6) | (7) |
| Management & Professional Liability | Operating Segments | Specialty               |            |   |   |
| Total revenues                                                                         | 2,776           | 2,577 | 2,572 |
| Surety | Operating Segments | Specialty                                                |            |   |   |
| Total revenues                                                                         | 604             | 596 | 596 |
| Warranty & Alternative Risks | Operating Segments | Specialty                      |            |   |   |
| Total revenues                                                                         | 1,624           | 1,412 | 1,323 |
| Middle Market | Operating Segments | Commercial                                        |            |   |   |
| Total revenues                                                                         | 1,508           | 1,444 | 1,439 |
| Construction | Operating Segments | Commercial                                         |            |   |   |
| Total revenues                                                                         | 1,322           | 1,120 | 1,043 |
| Small Business | Operating Segments | Commercial                                       |            |   |   |
| Total revenues                                                                         | 558             | 482 | 504 |
| Other Commercial | Operating Segments | Commercial                                     |            |   |   |
| Total revenues                                                                         | 811             | 815 | 810 |
| Canada | Operating Segments | International                                            |            |   |   |
| Total revenues                                                                         | 344             | 291 | 277 |
| Europe | Operating Segments | International                                            |            |   |   |
| Total revenues                                                                         | 473             | 389 | 363 |
| Hardy | Operating Segments | International                                             |            |   |   |
| Total revenues                                                                         | $ 297           | $ 318 | $ 397 |

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| - Definition
Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).
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