| **Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions** | 3 Months Ended | 6 Months Ended |
|----------------------------------------------------------------------------------------|----------------|----------------|
||
| Revenues                                                                               | $ 2,610        | $ 2,574        | $ 5,305 | $ 5,109 |
| Net investment gains (losses)                                                          | (18)           | (1)            | 13 | 11 |
| Operating Segments | Specialty                                                         |           |           |   |   |
| Revenues                                                                               | 1,106          | 1,061          | 2,204 | 2,094 |
| Operating Segments | Commercial                                                        |           |           |   |   |
| Revenues                                                                               | 920            | 918            | 1,880 | 1,818 |
| Operating Segments | International                                                     |           |           |   |   |
| Revenues                                                                               | 259            | 262            | 524 | 512 |
| Operating Segments | Life & Group                                                  |           |           |   |   |
| Revenues                                                                               | 335            | 329            | 670 | 664 |
| Operating Segments | Corporate & Other                                             |           |           |   |   |
| Revenues                                                                               | 9              | 5              | 18 | 11 |
| Eliminations                                                                           |           |           |   |   |
| Revenues                                                                               | (1)            | 0              | (4) | (1) |
| Total operating revenues                                                               |           |           |   |   |
| Revenues                                                                               | 2,628          | 2,575          | 5,292 | 5,098 |
| Management & Professional Liability | Operating Segments | Specialty               |           |           |   |   |
| Revenues                                                                               | 629            | 627            | 1,265 | 1,251 |
| Surety | Operating Segments | Specialty                                                |           |           |   |   |
| Revenues                                                                               | 151            | 145            | 290 | 274 |
| Warranty & Alternative Risks | Operating Segments | Specialty                      |           |           |   |   |
| Revenues                                                                               | 326            | 289            | 649 | 569 |
| Middle Market | Operating Segments | Commercial                                        |           |           |   |   |
| Revenues                                                                               | 545            | 521            | 1,092 | 1,025 |
| Small Business | Operating Segments | Commercial                                       |           |           |   |   |
| Revenues                                                                               | 115            | 120            | 236 | 239 |
| Other Commercial Insurance | Operating Segments | Commercial                           |           |           |   |   |
| Revenues                                                                               | 260            | 277            | 552 | 554 |
| Canada | Operating Segments | International                                            |           |           |   |   |
| Revenues                                                                               | 68             | 63             | 134 | 121 |
| Europe | Operating Segments | International                                            |           |           |   |   |
| Revenues                                                                               | 88             | 91             | 179 | 179 |
| Hardy | Operating Segments | International                                             |           |           |   |   |
| Revenues                                                                               | $ 103          | $ 108          | $ 211 | $ 212 |

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| - Definition
Investments and Derivatives, Realized Gain (Loss)
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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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