| **Reinsurance - Consolidated Property Casualty Insurance Assumed and Ceded Business (Details) - USD ($) $ in Millions** | 3 Months Ended |
|-------------------------------------------------------------------------------------------------------------------------|----------------|
||
| **Property Casualty Insurance Assumed And Ceded Business [Line Items]**                                                 |           |   |
| Incurred loss and loss expenses                                                                                         | $ 1,003        | $ 860 |
| Consolidated Property and Casualty Insurance Entity                                                                     |           |   |
| **Property Casualty Insurance Assumed And Ceded Business [Line Items]**                                                 |           |   |
| Direct written premiums                                                                                                 | 1,458          | 1,335 |
| Assumed written premiums                                                                                                | 111            | 87 |
| Ceded written premiums                                                                                                  | (51)           | (41) |
| Net written premiums                                                                                                    | 1,518          | 1,381 |
| Direct earned premiums                                                                                                  | 1,371          | 1,266 |
| Assumed earned premiums                                                                                                 | 66             | 43 |
| Ceded earned premiums                                                                                                   | (48)           | (42) |
| Earned premiums                                                                                                         | 1,389          | 1,267 |
| Direct incurred loss and loss expenses                                                                                  | 900            | 787 |
| Assumed incurred loss and loss expenses                                                                                 | 34             | 25 |
| Ceded incurred loss and loss expenses                                                                                   | (4)            | (22) |
| Incurred loss and loss expenses                                                                                         | $ 930          | $ 790 |

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[Line Items] for Property Casualty Insurance Premiums Written On Assumed And Ceded Business
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Amount of earned premiums assumed from other entities.
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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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Amount, after premiums ceded to other entities and premiums assumed by the entity, of property and casualty 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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