| **Reinsurance Reinsurance (Written Consolidated Property Casualty Insurance Premiums On Assumed and Ceded Business) (Details) - USD ($) $ in Millions** | 12 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|
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| **SEC Schedule, 12-17, Insurance Companies, Reinsurance**                                                                                               |            |   |   |
| Direct premiums earned                                                                                                                                  | $ 7,801         | $ 7,056 | $ 6,366 |
| Assumed premiums earned                                                                                                                                 | 569             | 561 | 443 |
| Ceded premiums earned                                                                                                                                   | (412)           | (392) | (331) |
| Incurred loss and loss expenses                                                                                                                         | 5,274           | 5,019 | 3,909 |
| Consolidated Property And Casualty Insurance Entity                                                                                                     |            |   |   |
| **SEC Schedule, 12-17, Insurance Companies, Reinsurance**                                                                                               |            |   |   |
| Direct premiums written                                                                                                                                 | 7,784           | 7,002 | 6,229 |
| Assumed premiums written                                                                                                                                | 597             | 619 | 515 |
| Ceded premiums written                                                                                                                                  | (335)           | (314) | (265) |
| Net written premiums                                                                                                                                    | 8,046           | 7,307 | 6,479 |
| Direct premiums earned                                                                                                                                  | 7,407           | 6,677 | 5,996 |
| Assumed premiums earned                                                                                                                                 | 569             | 561 | 443 |
| Ceded premiums earned                                                                                                                                   | (331)           | (314) | (255) |
| Earned premiums                                                                                                                                         | 7,645           | 6,924 | 6,184 |
| Direct incurred loss and loss expenses                                                                                                                  | 4,843           | 4,495 | 3,352 |
| Assumed incurred loss and loss expenses                                                                                                                 | 280             | 396 | 366 |
| Ceded incurred loss and loss expenses                                                                                                                   | (165)           | (175) | (122) |
| Incurred loss and loss expenses                                                                                                                         | $ 4,958         | $ 4,716 | $ 3,596 |

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Amount, after premiums ceded to other entities and premiums assumed by the entity, of property and casualty premiums written.
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Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums earned.
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Amount, before effect of policies assumed or ceded, of expense related to provision for policy benefits and costs incurred.
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