| **Property Casualty Loss and Loss Expenses  - Additional Information (Detail) - USD ($) $ in Millions** | 3 Months Ended | 6 Months Ended |         |         |         |         |
|---------------------------------------------------------------------------------------------------------|----------------|----------------|---------|---------|---------|---------|
||
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |           |    |    |    |    |   |   |
| Reserve for loss and loss expenses                                                                      | $ 5,954        | $ 5,423        | $ 5,954 | $ 5,423 | $ 5,886 | $ 5,646 | $ 5,293 | $ 5,219 |
| Prior accident years                                                                                    | (84)           | (31)           | (151)   | (79)    |    |    |   |   |
| Life and Health Loss Reserves                                                                           |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |           |    |    |    |    |   |   |
| Reserve for loss and loss expenses                                                                      | 55             | 45             | 55      | 45      |    |    |   |   |
| Commercial Lines                                                                                        |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    | (58)           | (42)           | (120)   | (77)    |    |    |   |   |
| Commercial Lines | Commercial Casualty Segment                                                          |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    | (25)           | (14)           | (56)    | (10)    |    |    |   |   |
| Commercial Lines | Workers' Compensation                                                                |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    | (27)           | (18)           | (42)    | (31)    |    |    |   |   |
| Commercial Lines | Commercial Auto                                                                      |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    | (6)            | 5              | (9)     | 4       |    |    |   |   |
| Commercial Lines | Other Commercial Lines                                                               |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    |           | (8)            |    | (12)    |    |    |   |   |
| Commercial Lines | Commercial Property Segment                                                          |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    |           | (7)            | (8)     | (28)    |    |    |   |   |
| Personal Lines                                                                                          |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    | (14)           | 17             | (11)    | 16      |    |    |   |   |
| Personal Lines | Homeowner                                                                              |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    |           | 14             | 15      | 17      |    |    |   |   |
| Personal Lines | Personal Auto                                                                          |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    | (15)           |           | (20)    |    |    |    |   |   |
| Excess and Surplus Lines Insurance                                                                      |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    | $ (5)          | $ (4)          | $ (7)   | (14)    |    |    |   |   |
| Consolidated Property and Casualty Insurance Entity | Catastrophe Development                           |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**             |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    |           |           |    | $ (7)   |    |    |   |   |

| X                                                                                                                                                                                                                                          |
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| - References
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| - Definition
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.
+ References
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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.
+ References
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