| **Reinsurance (Components of Earned Premiums) (Details) - USD ($) $ in Millions**                    | 12 Months Ended |
|------------------------------------------------------------------------------------------------------|-----------------|
||
| **Effects of Reinsurance [Line Items]**                                                              |            |   |   |
| Direct premiums earned                                                                               | $ 11,491        | $ 11,337 | $ 10,936 |
| Assumed premiums earned                                                                              | 338             | 355 | 367 |
| Ceded premiums earned                                                                                | 4,401           | 4,380 | 4,315 |
| Net earned premiums                                                                                  | 7,428           | 7,312 | 6,988 |
| Property and Casualty Insurance                                                                      |            |   |   |
| **Effects of Reinsurance [Line Items]**                                                              |            |   |   |
| Direct premiums earned                                                                               | 11,021          | 10,857 | 10,447 |
| Assumed premiums earned                                                                              | 288             | 305 | 317 |
| Ceded premiums earned                                                                                | 4,401           | 4,380 | 4,315 |
| Net earned premiums                                                                                  | 6,908           | 6,782 | 6,449 |
| Long term care Insurance Product Line                                                                |            |   |   |
| **Effects of Reinsurance [Line Items]**                                                              |            |   |   |
| Direct premiums earned                                                                               | 470             | 480 | 489 |
| Assumed premiums earned                                                                              | 50              | 50 | 50 |
| Ceded premiums earned                                                                                | 0               | 0 | 0 |
| Net earned premiums                                                                                  | $ 520           | $ 530 | $ 539 |
| Percentage of assumed premiums earned to net premiums earned                                         |            |   |   |
| **Effects of Reinsurance [Line Items]**                                                              |            |   |   |
| Percentage of assumed premium earned to premium earned net                                           | 4.60%           | 4.90% | 5.30% |
| Percentage of assumed premiums earned to net premiums earned | Property and Casualty Insurance       |            |   |   |
| **Effects of Reinsurance [Line Items]**                                                              |            |   |   |
| Percentage of assumed premium earned to premium earned net                                           | 4.20%           | 4.50% | 4.90% |
| Percentage of assumed premiums earned to net premiums earned | Long term care Insurance Product Line |            |   |   |
| **Effects of Reinsurance [Line Items]**                                                              |            |   |   |
| Percentage of assumed premium earned to premium earned net                                           | 9.60%           | 9.40% | 9.30% |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of earned premiums assumed from other entities.
+ References
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of earned premiums ceded to other entities.
+ References
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums earned.
+ References
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| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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 premium earned to premium earned after effects of reinsurance, as disclosed in supplementary reinsurance information.
+ References
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