| **Property Casualty Loss and Loss Expenses - Additional Information (Details) - USD ($) $ in Millions** | 3 Months Ended |         |         |
|---------------------------------------------------------------------------------------------------------|----------------|---------|---------|
||
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |    |    |   |
| Reserve for loss and loss expenses                                                                      | $ 8,626        | $ 7,287 | $ 8,336 | $ 7,229 |
| Prior accident years                                                                                    | (59)           | (41)    |    |   |
| Life and Health Loss Reserves                                                                           |           |    |    |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |    |    |   |
| Reserve for loss and loss expenses                                                                      | 67             | 79      |    |   |
| Commercial Insurance                                                                                    |           |    |    |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |    |    |   |
| Prior accident years                                                                                    | (32)           | (18)    |    |   |
| Commercial Insurance | Commercial Property Line                                                         |           |    |    |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |    |    |   |
| Prior accident years                                                                                    | (16)           |    |    |   |
| Commercial Insurance | Workers' Compensation Line                                                       |           |    |    |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |    |    |   |
| Prior accident years                                                                                    | (15)           | (10)    |    |   |
| Commercial Insurance | Commercial Auto Line                                                             |           |    |    |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |    |    |   |
| Prior accident years                                                                                    |           | (6)     |    |   |
| Personal Insurance                                                                                      |           |    |    |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |    |    |   |
| Prior accident years                                                                                    | (31)           | (34)    |    |   |
| Personal Insurance | Homeowner                                                                          |           |    |    |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |    |    |   |
| Prior accident years                                                                                    | (27)           | (31)    |    |   |
| Excess and Surplus Lines Insurance                                                                      |           |    |    |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |    |    |   |
| Prior accident years                                                                                    | $ (9)          | $ (5)   |    |   |

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| - References
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The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.
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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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