| **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                                                                        | $ 1,229        | $ 1,109        | $ 1,093 | $ 1,013 |
| Capitalized deferred policy acquisition costs                                                                                       | 447            | 365            | 1,351 | 1,166 |
| Amortized deferred policy acquisition costs                                                                                         | (435)          | (373)          | (1,203) | (1,078) |
| Deferred policy acquisition costs asset, end of period                                                                              | 1,241          | 1,101          | 1,241 | 1,101 |
| Consolidated Property And Casualty Insurance                                                                                        |           |           |   |   |
| **Movement Analysis of Deferred Policy Acquisition Costs**                                                                          |           |           |   |   |
| Deferred policy acquisition costs asset, beginning of period                                                                        | 878            | 771            | 749 | 682 |
| Capitalized deferred policy acquisition costs                                                                                       | 436            | 355            | 1,318 | 1,134 |
| Amortized deferred policy acquisition costs                                                                                         | (427)          | (366)          | (1,180) | (1,056) |
| Deferred policy acquisition costs asset, end of period                                                                              | 887            | 760            | 887 | 760 |
| Life Insurance Segment                                                                                                              |           |           |   |   |
| **Movement Analysis of Deferred Policy Acquisition Costs**                                                                          |           |           |   |   |
| Deferred policy acquisition costs asset, beginning of period                                                                        | 351            | 338            | 344 | 331 |
| Capitalized deferred policy acquisition costs                                                                                       | 11             | 10             | 33 | 32 |
| Amortized deferred policy acquisition costs                                                                                         | (8)            | (7)            | (23) | (22) |
| Deferred policy acquisition costs asset, end of period                                                                              | $ 354          | $ 341          | $ 354 | $ 341 |

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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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