| **Reinsurance - Consolidated Property Casualty Insurance Assumed and Ceded Business (Details) - USD ($) $ in Millions** | 3 Months Ended | 6 Months Ended |
|-------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Property Casualty Insurance Assumed And Ceded Business [Line Items]**                                                 |           |           |   |   |
| Incurred loss and loss expenses                                                                                         | $ 936          | $ 883          | $ 1,796 | $ 1,737 |
| Consolidated Property and Casualty Insurance Entity                                                                     |           |           |   |   |
| **Property Casualty Insurance Assumed And Ceded Business [Line Items]**                                                 |           |           |   |   |
| Direct written premiums                                                                                                 | 1,472          | 1,336          | 2,807 | 2,583 |
| Assumed written premiums                                                                                                | 75             | 52             | 162 | 101 |
| Ceded written premiums                                                                                                  | (71)           | (39)           | (112) | (77) |
| Net written premiums                                                                                                    | 1,476          | 1,349          | 2,857 | 2,607 |
| Direct earned premiums                                                                                                  | 1,319          | 1,235          | 2,585 | 2,442 |
| Assumed earned premiums                                                                                                 | 50             | 36             | 93 | 69 |
| Ceded earned premiums                                                                                                   | (52)           | (41)           | (94) | (81) |
| Earned premiums                                                                                                         | 1,317          | 1,230          | 2,584 | 2,430 |
| Direct incurred loss and loss expenses                                                                                  | 866            | 821            | 1,653 | 1,602 |
| Assumed incurred loss and loss expenses                                                                                 | 25             | 13             | 50 | 29 |
| Ceded incurred loss and loss expenses                                                                                   | (28)           | (13)           | (50) | (19) |
| Incurred loss and loss expenses                                                                                         | $ 863          | $ 821          | $ 1,653 | $ 1,612 |

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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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