| **Reinsurance - Consolidated Property Casualty Insurance Assumed and Ceded Business (Details) - USD ($) $ in Millions** | 3 Months Ended |
|-------------------------------------------------------------------------------------------------------------------------|----------------|
||
| **Property Casualty Insurance Assumed And Ceded Business**                                                              |           |   |
| Earned premiums                                                                                                         | $ 1,918        | $ 1,693 |
| Incurred loss and loss expenses                                                                                         | 1,398          | 1,032 |
| Consolidated Property And Casualty Insurance                                                                            |           |   |
| **Property Casualty Insurance Assumed And Ceded Business**                                                              |           |   |
| Direct written premiums                                                                                                 | 1,859          | 1,703 |
| Assumed written premiums                                                                                                | 244            | 263 |
| Ceded written premiums                                                                                                  | (84)           | (67) |
| Net written premiums                                                                                                    | 2,019          | 1,899 |
| Direct earned premiums                                                                                                  | 1,760          | 1,561 |
| Assumed earned premiums                                                                                                 | 159            | 121 |
| Ceded earned premiums                                                                                                   | (78)           | (64) |
| Earned premiums                                                                                                         | 1,841          | 1,618 |
| Direct incurred loss and loss expenses                                                                                  | 1,299          | 895 |
| Assumed incurred loss and loss expenses                                                                                 | 76             | 73 |
| Ceded incurred loss and loss expenses                                                                                   | (58)           | (12) |
| Incurred loss and loss expenses                                                                                         | $ 1,317        | $ 956 |

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[Line Items] for Property Casualty Insurance Premiums Written On Assumed And Ceded Business
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| - Definition
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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| - Definition
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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| - Definition
Amount of reduction of provision for policy benefits and costs incurred for policies ceded.
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