| **Supplementary Insurance Information (Details) (USD $)In Millions, unless otherwise specified** | 12 Months Ended |
|--------------------------------------------------------------------------------------------------|-----------------|
||
| **Supplementary Insurance Information, by Segment [Line Items]**                                 |            |   |   |
| Deferred policy acquisition costs                                                                | $ 565           | $ 470 | $ 477 |
| Gross future policy benefits, losses, claims and expense losses                                  | 6,682           | 6,510 | 6,537 |
| Gross unearned premium                                                                           | 1,976           | 1,792 | 1,633 |
| Other policy claims and benefits payable                                                         | 19              | 15 | 16 |
| Earned premiums                                                                                  | 3,902           | 3,522 | 3,194 |
| Investment income, net of expenses                                                               | 488             | 488 | 484 |
| Benefits, claims losses and settlement expense                                                   | 2,505           | 2,322 | 2,524 |
| Amortization of deferred policy acquisition costs                                                | 758             | 707 | 631 |
| Underwriting, acquisition and insurance expenses                                                 | 485             | 448 | 408 |
| Net written premiums                                                                             | 3,895           | 3,485 | 3,101 |
| Commercial Lines [Member]                                                                        |            |   |   |
| **Supplementary Insurance Information, by Segment [Line Items]**                                 |            |   |   |
| Deferred policy acquisition costs                                                                | 251             | 235 | 204 |
| Gross future policy benefits, losses, claims and expense losses                                  | 3,667           | 3,645 | 3,780 |
| Gross unearned premium                                                                           | 1,372           | 1,246 | 1,149 |
| Earned premiums                                                                                  | 2,636           | 2,383 | 2,197 |
| Benefits, claims losses and settlement expense                                                   | 1,596           | 1,420 | 1,570 |
| Amortization of deferred policy acquisition costs                                                | 514             | 462 | 441 |
| Underwriting, acquisition and insurance expenses                                                 | 343             | 324 | 291 |
| Net written premiums                                                                             | 2,760           | 2,459 | 2,218 |
| Personal Lines [Member]                                                                          |            |   |   |
| **Supplementary Insurance Information, by Segment [Line Items]**                                 |            |   |   |
| Deferred policy acquisition costs                                                                | 104             | 93 | 92 |
| Gross future policy benefits, losses, claims and expense losses                                  | 417             | 398 | 419 |
| Gross unearned premium                                                                           | 535             | 490 | 440 |
| Earned premiums                                                                                  | 961             | 868 | 762 |
| Benefits, claims losses and settlement expense                                                   | 639             | 652 | 723 |
| Amortization of deferred policy acquisition costs                                                | 192             | 183 | 144 |
| Underwriting, acquisition and insurance expenses                                                 | 98              | 78 | 78 |
| Net written premiums                                                                             | 1,005           | 918 | 801 |
| Excess And Surplus Lines Insurance [Member]                                                      |            |   |   |
| **Supplementary Insurance Information, by Segment [Line Items]**                                 |            |   |   |
| Deferred policy acquisition costs                                                                | 11              | 9 | 7 |
| Gross future policy benefits, losses, claims and expense losses                                  | 157             | 126 | 81 |
| Gross unearned premium                                                                           | 67              | 54 | 42 |
| Earned premiums                                                                                  | 116             | 93 | 70 |
| Benefits, claims losses and settlement expense                                                   | 66              | 65 | 42 |
| Amortization of deferred policy acquisition costs                                                | 21              | 17 | 13 |
| Underwriting, acquisition and insurance expenses                                                 | 15              | 12 | 9 |
| Net written premiums                                                                             | 128             | 105 | 79 |
| Property, Liability and Casualty Insurance Product Line [Member]                                 |            |   |   |
| **Supplementary Insurance Information, by Segment [Line Items]**                                 |            |   |   |
| Deferred policy acquisition costs                                                                | 366             | 337 | 303 |
| Gross future policy benefits, losses, claims and expense losses                                  | 4,241           | 4,169 | 4,280 |
| Gross unearned premium                                                                           | 1,974           | 1,790 | 1,631 |
| Earned premiums                                                                                  | 3,713           | 3,344 | 3,029 |
| Investment income, net of expenses                                                               | 348             | 351 | 350 |
| Benefits, claims losses and settlement expense                                                   | 2,301           | 2,137 | 2,335 |
| Amortization of deferred policy acquisition costs                                                | 727             | 662 | 598 |
| Underwriting, acquisition and insurance expenses                                                 | 456             | 414 | 378 |
| Net written premiums                                                                             | 3,893           | 3,482 | 3,098 |
| Accident Health Insurance Segment [Member]                                                       |            |   |   |
| **Supplementary Insurance Information, by Segment [Line Items]**                                 |            |   |   |
| Net written premiums                                                                             | 2               | 3 | 3 |
| Life Insurance Segment [Member]                                                                  |            |   |   |
| **Supplementary Insurance Information, by Segment [Line Items]**                                 |            |   |   |
| Deferred policy acquisition costs                                                                | 199             | 133 | 174 |
| Gross future policy benefits, losses, claims and expense losses                                  | 2,441           | 2,341 | 2,257 |
| Gross unearned premium                                                                           | 2               | 2 | 2 |
| Other policy claims and benefits payable                                                         | 19              | 15 | 16 |
| Earned premiums                                                                                  | 189             | 178 | 165 |
| Investment income, net of expenses                                                               | 140             | 137 | 134 |
| Benefits, claims losses and settlement expense                                                   | 204             | 185 | 189 |
| Amortization of deferred policy acquisition costs                                                | 31              | 45 | 33 |
| Underwriting, acquisition and insurance expenses                                                 | $ 29            | $ 34 | $ 30 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of amortization of deferred policy acquisition costs recognized.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of insurance benefits, claims, losses and settlement expenses incurred.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net amount of deferred policy acquisition costs capitalized on contracts remaining in force on the balance sheet date.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of reserves for future policy claims payable and loss expenses to be incurred.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of net investment income earned.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of other operating expense incurred.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of claims and benefits payable not included within other categories.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of premium revenue earned.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of actual written premiums, including, as applicable, premiums from reinsurance assumed and net of premiums on reinsurance ceded.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of premiums not yet earned.
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