| **Supplementary Information Concerning Property Casualty Insurance Operations (Details) (USD $)In Millions, unless otherwise specified** | 12 Months Ended |
|------------------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                                                  |            |   |   |
| Deferred policy acquisition costs                                                                                                        | $ 366           | $ 337 | $ 303 |
| Reserves for unpaid claims and claim adjustment expenses                                                                                 | 4,241           | 4,169 | 4,280 |
| Unearned premiums                                                                                                                        | 1,970           | 1,790 | 1,631 |
| Earned premiums                                                                                                                          | 3,713           | 3,344 | 3,029 |
| Investment income                                                                                                                        | 348             | 351 | 350 |
| Loss and loss expenses incurred related to current accident year                                                                         | 2,448           | 2,533 | 2,620 |
| Loss and loss expenses incurred related to prior accident year                                                                           | 147             | 396 | 285 |
| Amortization of deferred policy acquisition cost                                                                                         | 727             | 661 | 598 |
| Paid loss and loss expenses                                                                                                              | 2,172           | 2,229 | 2,241 |
| Net written premiums                                                                                                                     | 3,893           | 3,482 | 3,098 |
| Commercial Lines [Member]                                                                                                                |            |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                                                  |            |   |   |
| Deferred policy acquisition costs                                                                                                        | 251             | 235 | 204 |
| Reserves for unpaid claims and claim adjustment expenses                                                                                 | 3,667           | 3,645 | 3,780 |
| Unearned premiums                                                                                                                        | 1,370           | 1,246 | 1,149 |
| Earned premiums                                                                                                                          | 2,636           | 2,383 | 2,197 |
| Investment income                                                                                                                        | 0               | 0 | 0 |
| Loss and loss expenses incurred related to current accident year                                                                         | 1,691           | 1,712 | 1,804 |
| Loss and loss expenses incurred related to prior accident year                                                                           | 95              | 292 | 234 |
| Amortization of deferred policy acquisition cost                                                                                         | 514             | 461 | 441 |
| Paid loss and loss expenses                                                                                                              | 1,498           | 1,546 | 1,545 |
| Net written premiums                                                                                                                     | 2,760           | 2,459 | 2,218 |
| Personal Lines [Member]                                                                                                                  |            |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                                                  |            |   |   |
| Deferred policy acquisition costs                                                                                                        | 104             | 93 | 92 |
| Reserves for unpaid claims and claim adjustment expenses                                                                                 | 417             | 398 | 419 |
| Unearned premiums                                                                                                                        | 534             | 490 | 440 |
| Earned premiums                                                                                                                          | 961             | 868 | 762 |
| Investment income                                                                                                                        | 0               | 0 | 0 |
| Loss and loss expenses incurred related to current accident year                                                                         | 678             | 751 | 765 |
| Loss and loss expenses incurred related to prior accident year                                                                           | 39              | 99 | 42 |
| Amortization of deferred policy acquisition cost                                                                                         | 192             | 183 | 144 |
| Paid loss and loss expenses                                                                                                              | 639             | 659 | 676 |
| Net written premiums                                                                                                                     | 1,005           | 918 | 801 |
| Excess And Surplus Lines Insurance [Member]                                                                                              |            |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                                                  |            |   |   |
| Deferred policy acquisition costs                                                                                                        | 11              | 9 | 7 |
| Reserves for unpaid claims and claim adjustment expenses                                                                                 | 157             | 126 | 81 |
| Unearned premiums                                                                                                                        | 66              | 54 | 42 |
| Earned premiums                                                                                                                          | 116             | 93 | 70 |
| Investment income                                                                                                                        | 0               | 0 | 0 |
| Loss and loss expenses incurred related to current accident year                                                                         | 79              | 70 | 51 |
| Loss and loss expenses incurred related to prior accident year                                                                           | 13              | 5 | 9 |
| Amortization of deferred policy acquisition cost                                                                                         | 21              | 17 | 13 |
| Paid loss and loss expenses                                                                                                              | 35              | 24 | 20 |
| Net written premiums                                                                                                                     | $ 128           | $ 105 | $ 79 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of amortization expense on deferred policy acquisition cost.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of insurance benefit, claims, loss and settlement expenses paid to the policyholders during the current accounting period.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of deferred policy acquisition cost related to property-casualty insurance policy written.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of property-casualty insurance premium revenue earned on insurance policies.
+ References
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| X                                                                                                                                                                                                            |
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of net investment income earned by property-casualty insurance underwriter.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Payments of claims and claims adjustment expenses during the period of a property-casualty insurance entity.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of actual premium written by property-casualty insurance underwriter.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of insurance benefit, claims, loss and settlement expenses paid to the policyholders during the prior accounting period.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of reserve, policy claim payable and loss expense incurred by property-casualty insurance underwriter.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of property-casualty insurance premium revenue not yet earned on insurance policies.
+ References
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