| **Reinsurance (Components of Earned Premiums) (Details) - USD ($) $ in Millions**                    | 12 Months Ended |
|------------------------------------------------------------------------------------------------------|-----------------|
||
| **Effects of Reinsurance [Line Items]**                                                              |            |   |   |
| Direct premiums earned                                                                               | $ 10,936        | $ 10,886 | $ 10,351 |
| Assumed premiums earned                                                                              | 367             | 308 | 324 |
| Ceded premiums earned                                                                                | 4,315           | 4,270 | 3,754 |
| Net earned premiums                                                                                  | 6,988           | 6,924 | 6,921 |
| Property and Casualty Insurance                                                                      |            |   |   |
| **Effects of Reinsurance [Line Items]**                                                              |            |   |   |
| Direct premiums earned                                                                               | 10,447          | 10,400 | 9,853 |
| Assumed premiums earned                                                                              | 317             | 258 | 274 |
| Ceded premiums earned                                                                                | 4,315           | 4,270 | 3,754 |
| Net earned premiums                                                                                  | 6,449           | 6,388 | 6,373 |
| Long term care Insurance Product Line                                                                |            |   |   |
| **Effects of Reinsurance [Line Items]**                                                              |            |   |   |
| Direct premiums earned                                                                               | 489             | 486 | 498 |
| Assumed premiums earned                                                                              | 50              | 50 | 50 |
| Ceded premiums earned                                                                                | 0               | 0 | 0 |
| Net earned premiums                                                                                  | $ 539           | $ 536 | $ 548 |
| Percentage of assumed premiums earned to net premiums earned                                         |            |   |   |
| **Effects of Reinsurance [Line Items]**                                                              |            |   |   |
| Percentage of assumed premium earned to premium earned net                                           | 5.30%           | 4.40% | 4.70% |
| 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                                           | 4.90%           | 4.00% | 4.30% |
| 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                                           | 9.30%           | 9.30% | 9.10% |

| 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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| 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
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 premiums earned by the entity to net premiums earned.
+ References
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