| **Schedule VI. Supplemental Information Concerning Property and Casualty Insurance Operations (Details) - USD ($) $ in Millions** | 12 Months Ended |
|-----------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| Incurred claim and claim adjustment expenses related to current year                                                              | $ 5,970         | $ 5,793 | $ 5,356 |
| Incurred claim and claim adjustment expenses related to prior years                                                               | (104)           | (119) | (127) |
| Consolidated Property and Casualty Insurance Entity                                                                               |            |   |   |
| Deferred acquisition costs                                                                                                        | 737             | 708 |   |
| Reserves for unpaid claim and claim adjustment expenses                                                                           | 24,174          | 22,706 |   |
| Discount deducted from claim and claim adjustment expense reserves above (based on interest rates ranging from 3.5% to 6.4%)      | 1,146           | 1,209 |   |
| Unearned premiums                                                                                                                 | 5,761           | 5,119 |   |
| Net written premiums                                                                                                              | 8,405           | 8,059 | 7,656 |
| Net earned premiums                                                                                                               | 8,175           | 7,649 | 7,428 |
| Net investment income                                                                                                             | 2,111           | 1,896 | 2,063 |
| Incurred claim and claim adjustment expenses related to current year                                                              |            | 5,793 | 5,356 |
| Incurred claim and claim adjustment expenses related to prior years                                                               |            | (119) | (127) |
| Amortization of deferred acquisition costs                                                                                        | $ 1,443         | 1,410 | 1,383 |
| Paid claim and claim adjustment expenses                                                                                          |            | $ 5,164 | $ 5,576 |
| Consolidated Property and Casualty Insurance Entity | Minimum                                                                     |            |   |   |
| **Supplemental Information Parenthetical**                                                                                        |            |   |   |
| SEC Schedule, 12-18, supplemental information, property-casualty insurance underwriters, interest rate at which discount computed | 3.50%           | 3.50% |   |
| Consolidated Property and Casualty Insurance Entity | Maximum                                                                     |            |   |   |
| **Supplemental Information Parenthetical**                                                                                        |            |   |   |
| SEC Schedule, 12-18, supplemental information, property-casualty insurance underwriters, interest rate at which discount computed | 6.40%           | 6.40% |   |

| X                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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Schedule Vi Supplemental Information Parenthetical [Abstract]
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| - Definition
Amount of amortization expense on deferred policy acquisition cost for property-casualty insurance underwriter.
+ References
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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.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of deferred policy acquisition cost related to property-casualty insurance policy written.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of discount deducted from reserve for unpaid claim and claim adjustment expense by property-casualty insurance underwriter.
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Amount of premium revenue earned by property-casualty insurance underwriter.
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| - Definition
Rate at which discount was computed for deduction from reserve for unpaid claim and claim adjustment expense by property-casualty insurance underwriter.
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Amount of net investment income earned by property-casualty insurance underwriter.
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| - Definition
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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