| **Reinsurance - Consolidated Property Casualty Insurance Assumed and Ceded Business (Details) - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |
|-------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Property Casualty Insurance Assumed And Ceded Business**                                                              |           |           |   |   |
| Earned premiums                                                                                                         | $ 2,297        | $ 2,033        | $ 6,524 | $ 5,894 |
| Incurred loss and loss expenses                                                                                         | 1,578          | 1,332          | 4,407 | 4,070 |
| Consolidated Property And Casualty Insurance                                                                            |           |           |   |   |
| **Property Casualty Insurance Assumed And Ceded Business**                                                              |           |           |   |   |
| Direct written premiums                                                                                                 | 2,285          | 1,939          | 6,772 | 5,869 |
| Assumed written premiums                                                                                                | 102            | 89             | 577 | 527 |
| Ceded written premiums                                                                                                  | (94)           | (71)           | (349) | (270) |
| Net written premiums                                                                                                    | 2,293          | 1,957          | 7,000 | 6,126 |
| Direct earned premiums                                                                                                  | 2,179          | 1,912          | 6,128 | 5,478 |
| Assumed earned premiums                                                                                                 | 159            | 142            | 466 | 435 |
| Ceded earned premiums                                                                                                   | (121)          | (97)           | (310) | (252) |
| Earned premiums                                                                                                         | 2,217          | 1,957          | 6,284 | 5,661 |
| Direct incurred loss and loss expenses                                                                                  | 1,415          | 1,220          | 3,960 | 3,784 |
| Assumed incurred loss and loss expenses                                                                                 | 103            | 82             | 242 | 216 |
| Ceded incurred loss and loss expenses                                                                                   | (19)           | (41)           | (21) | (160) |
| Incurred loss and loss expenses                                                                                         | $ 1,499        | $ 1,261        | $ 4,181 | $ 3,840 |

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Amount of earned premiums assumed from other entities.
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| - Definition
Amount of earned premiums ceded to other entities.
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| - Definition
Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums earned.
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Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums written.
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The increase (decrease) in property and casualty insurance liabilities balances during the period.
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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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Amount, after effects of policies assumed or ceded, of expense related to the provision for policy benefits and costs incurred.
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| - Definition
Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.
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Amount of policy benefits and costs incurred for policies assumed.
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Amount of reduction of provision for policy benefits and costs incurred for policies ceded.
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