| **Reinsurance (Components of Earned Premiums) (Details) - USD ($) $ in Millions**                                | 12 Months Ended |
|------------------------------------------------------------------------------------------------------------------|-----------------|
||
| Direct premiums earned                                                                                           | $ 10,351        | $ 9,960 | $ 9,574 |
| Assumed premiums earned                                                                                          | 324             | 325 | 306 |
| Ceded premiums earned                                                                                            | 3,754           | 3,073 | 2,609 |
| Net earned premiums                                                                                              | 6,921           | 7,212 | 7,271 |
| Property and Casualty Insurance [Member]                                                                         |            |   |   |
| Direct premiums earned                                                                                           | 9,853           | 9,452 | 9,063 |
| Assumed premiums earned                                                                                          | 274             | 277 | 258 |
| Ceded premiums earned                                                                                            | 3,754           | 3,073 | 2,609 |
| Net earned premiums                                                                                              | 6,373           | 6,656 | 6,712 |
| Accident and Health Insurance [Member]                                                                           |            |   |   |
| Direct premiums earned                                                                                           | 498             | 508 | 511 |
| Assumed premiums earned                                                                                          | 50              | 48 | 48 |
| Ceded premiums earned                                                                                            | 0               | 0 | 0 |
| Net earned premiums                                                                                              | $ 548           | $ 556 | $ 559 |
| Percentage of assumed premiums earned to net premiums earned [Member]                                            |            |   |   |
| Percentage of assumed premium earned to premium earned net                                                       | 4.70%           | 4.50% | 4.20% |
| Percentage of assumed premiums earned to net premiums earned [Member] | Property and Casualty Insurance [Member] |            |   |   |
| Percentage of assumed premium earned to premium earned net                                                       | 4.30%           | 4.20% | 3.80% |
| Percentage of assumed premiums earned to net premiums earned [Member] | Accident and Health Insurance [Member]   |            |   |   |
| Percentage of assumed premium earned to premium earned net                                                       | 9.10%           | 8.60% | 8.60% |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of earned premiums assumed from other entities.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of earned premiums ceded to other entities.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums earned.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of assumed premiums earned by the entity to net premiums earned.
+ References
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| X                                                                                                                                                                                                                       |
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| X                                                                                                                                                                                                            |
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| X                                                                                                                                                                                                                   |
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