| **Deferred Policy Acquisition Costs - Deferred Policy Acquisition Costs and Asset Reconciliation (Detail) - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |
|-------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
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| **Movement Analysis of Deferred Policy Acquisition Costs**                                                                          |           |           |   |   |
| Deferred policy acquisition costs asset, beginning of period                                                                        | $ 1,109        | $ 1,046        | $ 1,013 | $ 916 |
| Capitalized deferred policy acquisition costs                                                                                       | 365            | 336            | 1,166 | 1,097 |
| Amortized deferred policy acquisition costs                                                                                         | (373)          | (349)          | (1,078) | (980) |
| Deferred policy acquisition costs asset, end of period                                                                              | 1,101          | 1,033          | 1,101 | 1,033 |
| Consolidated Property And Casualty Insurance                                                                                        |           |           |   |   |
| **Movement Analysis of Deferred Policy Acquisition Costs**                                                                          |           |           |   |   |
| Deferred policy acquisition costs asset, beginning of period                                                                        | 771            | 723            | 682 | 602 |
| Capitalized deferred policy acquisition costs                                                                                       | 355            | 325            | 1,134 | 1,064 |
| Amortized deferred policy acquisition costs                                                                                         | (366)          | (342)          | (1,056) | (960) |
| Deferred policy acquisition costs asset, end of period                                                                              | 760            | 706            | 760 | 706 |
| Life Insurance Segment                                                                                                              |           |           |   |   |
| **Movement Analysis of Deferred Policy Acquisition Costs**                                                                          |           |           |   |   |
| Deferred policy acquisition costs asset, beginning of period                                                                        | 338            | 323            | 331 | 314 |
| Capitalized deferred policy acquisition costs                                                                                       | 10             | 11             | 32 | 33 |
| Amortized deferred policy acquisition costs                                                                                         | (7)            | (7)            | (22) | (20) |
| Deferred policy acquisition costs asset, end of period                                                                              | $ 341          | $ 327          | $ 341 | $ 327 |

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| - Definition
Amount of amortization expense (reversal of expense) for deferred policy acquisition costs.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of deferred policy acquisition cost capitalized on contract remaining in force.
+ References
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of deferred policy acquisition cost capitalized.
+ References
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| - Definition
A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.
+ References
No definition available.
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