| **Supplementary Information Concerning Property Casualty Insurance Operations (Details) - USD ($) $ in Millions** | 12 Months Ended |
|-------------------------------------------------------------------------------------------------------------------|-----------------|
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| **Property Casualty Insurance Underwriters**                                                                      |            |   |   |
| Deferred policy acquisition costs                                                                                 | $ 749           | $ 682 | $ 602 |
| Reserves for unpaid claims and claim adjustment expenses                                                          | 8,975           | 8,336 | 7,229 |
| Unearned premiums                                                                                                 | 4,118           | 3,688 | 3,270 |
| Earned premiums                                                                                                   | 7,645           | 6,924 | 6,184 |
| Investment income                                                                                                 | 602             | 509 | 457 |
| Loss and loss expenses incurred related to current accident year                                                  | 5,173           | 4,875 | 4,024 |
| Prior year claims and claims adjustments expense, (favorable) and unfavorable                                     | (215)           | (159) | (428) |
| Amortization of deferred policy acquisition cost                                                                  | 1,421           | 1,303 | 1,151 |
| Paid loss and loss expenses                                                                                       | 4,276           | 3,687 | 3,094 |
| Net written premiums                                                                                              | 8,046           | 7,307 | 6,479 |
| Commercial Insurance                                                                                              |            |   |   |
| **Property Casualty Insurance Underwriters**                                                                      |            |   |   |
| Deferred policy acquisition costs                                                                                 | 387             | 372 | 345 |
| Reserves for unpaid claims and claim adjustment expenses                                                          | 5,887           | 5,568 | 5,007 |
| Unearned premiums                                                                                                 | 2,111           | 2,021 | 1,857 |
| Earned premiums                                                                                                   | 4,264           | 4,024 | 3,674 |
| Investment income                                                                                                 | 0               | 0 | 0 |
| Loss and loss expenses incurred related to current accident year                                                  | 2,910           | 2,837 | 2,293 |
| Prior year claims and claims adjustments expense, (favorable) and unfavorable                                     | (123)           | (76) | (353) |
| Amortization of deferred policy acquisition cost                                                                  | 808             | 769 | 703 |
| Paid loss and loss expenses                                                                                       | 2,480           | 2,211 | 1,806 |
| Net written premiums                                                                                              | 4,336           | 4,159 | 3,811 |
| Personal Insurance                                                                                                |            |   |   |
| **Property Casualty Insurance Underwriters**                                                                      |            |   |   |
| Deferred policy acquisition costs                                                                                 | 210             | 168 | 145 |
| Reserves for unpaid claims and claim adjustment expenses                                                          | 990             | 916 | 814 |
| Unearned premiums                                                                                                 | 1,253           | 994 | 850 |
| Earned premiums                                                                                                   | 2,044           | 1,689 | 1,542 |
| Investment income                                                                                                 | 0               | 0 | 0 |
| Loss and loss expenses incurred related to current accident year                                                  | 1,506           | 1,227 | 1,042 |
| Prior year claims and claims adjustments expense, (favorable) and unfavorable                                     | (64)            | (61) | (50) |
| Amortization of deferred policy acquisition cost                                                                  | 367             | 311 | 276 |
| Paid loss and loss expenses                                                                                       | 1,353           | 1,071 | 914 |
| Net written premiums                                                                                              | 2,302           | 1,831 | 1,594 |
| Excess and Surplus Lines Insurance                                                                                |            |   |   |
| **Property Casualty Insurance Underwriters**                                                                      |            |   |   |
| Deferred policy acquisition costs                                                                                 | 43              | 37 | 33 |
| Reserves for unpaid claims and claim adjustment expenses                                                          | 932             | 753 | 577 |
| Unearned premiums                                                                                                 | 273             | 236 | 207 |
| Earned premiums                                                                                                   | 542             | 485 | 398 |
| Investment income                                                                                                 | 0               | 0 | 0 |
| Loss and loss expenses incurred related to current accident year                                                  | 361             | 324 | 243 |
| Prior year claims and claims adjustments expense, (favorable) and unfavorable                                     | (11)            | (9) | 7 |
| Amortization of deferred policy acquisition cost                                                                  | 89              | 79 | 63 |
| Paid loss and loss expenses                                                                                       | 183             | 145 | 118 |
| Net written premiums                                                                                              | 570             | 502 | 426 |
| Reinsurance assumed and other non segment                                                                         |            |   |   |
| **Property Casualty Insurance Underwriters**                                                                      |            |   |   |
| Deferred policy acquisition costs                                                                                 | 109             | 105 | 79 |
| Reserves for unpaid claims and claim adjustment expenses                                                          | 1,166           | 1,099 | 831 |
| Unearned premiums                                                                                                 | 481             | 437 | 356 |
| Earned premiums                                                                                                   | 795             | 726 | 570 |
| Investment income                                                                                                 | 0               | 0 | 0 |
| Loss and loss expenses incurred related to current accident year                                                  | 396             | 487 | 446 |
| Prior year claims and claims adjustments expense, (favorable) and unfavorable                                     | (17)            | (13) | (32) |
| Amortization of deferred policy acquisition cost                                                                  | 157             | 144 | 109 |
| Paid loss and loss expenses                                                                                       | 260             | 260 | 256 |
| Net written premiums                                                                                              | $ 838           | $ 815 | $ 648 |

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| - Definition
Amount of amortization expense on deferred policy acquisition cost for property-casualty insurance underwriter.
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| - Definition
Amount, after effects of reinsurance, of expense for claims incurred in the current reporting period and related claims settlement costs.
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| - Definition
Amount of deferred policy acquisition cost related to property-casualty insurance policy written.
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| - Definition
Amount of premium revenue earned by property-casualty insurance underwriter.
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Amount of net investment income earned by property-casualty insurance underwriter.
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Amount of loss reported and paid for claim and claim adjustment expense by property-casualty insurance underwriter.
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Amount of premium written by property-casualty insurance underwriter.
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| - Definition
Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs.
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Amount of reserve, policy claim payable and loss expense incurred by property-casualty insurance underwriter.
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Amount of premium written but not yet earned by property-casualty insurance underwriter.
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