| **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                                                                                                         | $ 1,690        | $ 1,544 |
| Incurred loss and loss expenses                                                                                         | 1,039          | 1,003 |
| Consolidated Property And Casualty Insurance                                                                            |           |   |
| **Property Casualty Insurance Assumed And Ceded Business**                                                              |           |   |
| Direct written premiums                                                                                                 | 1,703          | 1,545 |
| Assumed written premiums                                                                                                | 263            | 205 |
| Ceded written premiums                                                                                                  | (67)           | (57) |
| Net written premiums                                                                                                    | 1,899          | 1,693 |
| Direct earned premiums                                                                                                  | 1,561          | 1,429 |
| Assumed earned premiums                                                                                                 | 121            | 101 |
| Ceded earned premiums                                                                                                   | (64)           | (55) |
| Earned premiums                                                                                                         | 1,618          | 1,475 |
| Direct incurred loss and loss expenses                                                                                  | 895            | 868 |
| Assumed incurred loss and loss expenses                                                                                 | 73             | 78 |
| Ceded incurred loss and loss expenses                                                                                   | (12)           | (23) |
| Incurred loss and loss expenses                                                                                         | $ 956          | $ 923 |

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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
[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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Amount of written 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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| - Definition
Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums written.
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| - Definition
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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| - Definition
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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