| **Investments - Additional Information (Detail) $ in Millions**                                 | 3 Months Ended | 12 Months Ended |
|-------------------------------------------------------------------------------------------------|----------------|-----------------|
||
| **Gain (Loss) on Securities**                                                                   |           |            |   |
| Equity Securities at Fair Value                                                                 | $ 10,675       |            | $ 11,315 |
| Largest Single Common Stock Holding                                                             | $ 848          |            | $ 862 |
| Largest equity holdings to total equity portfolio                                               | 8.30%          |            | 7.90% |
| Largest equity holdings to total portfolio                                                      | 3.70%          |            | 3.50% |
| Maximum                                                                                         |           |            |   |
| **Gain (Loss) on Securities**                                                                   |           |            |   |
| Debt Securities, Available-for-sale, Allowance for Credit Loss                                  | $ 1            |            | $ 1 |
| Writedown of Impaired Securities Intended to be Sold                                            | 1              |            |   |
| Changes in Allowance for Credit Losses During Period                                            | $ 1            |            | $ 1 |
| Fixed maturities                                                                                |           |            |   |
| **Gain (Loss) on Securities**                                                                   |           |            |   |
| Number Of Impaired Securities Intended to be Sold | security                                    | 1              | 0               |   |
| Number Of Securities Continuous Unrealized Loss Position | security                             | 1,377          |            | 278 |
| Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss 3            | $ 245          |            | $ 16 |
| Number Of Securities Continuous Unrealized Loss Position and Below 70 Percent of Amortized Cost | 0              |            | 0 |
| Fixed maturity investments, Percentage of amortized costs                                       | 70.00%         |            | 70.00% |

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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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Amount of unrealized loss on securities classified as available-for-sale.
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| X                                                                                                                                                                                                                                                                                        |
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Changes in Allowance for Credit Losses During Period
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| X                                                                                                                                                                                                                                                                                                   |
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| - Definition
Fixed maturity investments, Percentage of amortized costs.
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| X                                                                                                                                                                                                                                                        |
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| - Definition
Largest Single Common Stock Holding
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| X                                                                                                                                                                                                                                                                                  |
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| - Definition
largest equity holdings to total equity portfolio
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| X                                                                                                                                                                                                                                                                     |
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Largest equity holdings to total portfolio
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| X                                                                                                                                                                                                                                                                                                                                     |
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| - Definition
Number of impaired available-for-sale debt securities intended to be sold and written down to fair value
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| X                                                                                                                                                                                                                                                                                            |
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| - Definition
Number Of Securities Continuous Unrealized Loss Position
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| - Definition
Number Of Securities Continuous Unrealized Loss Position and Below 70 Percent of Amortized Cost
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| X                                                                                                                                                                                                                                                                                                                                |
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| - Definition
Writedown of impaired available-for-sale debt securities to fair value due to intent to sell
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| - Definition
Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in marketable security.
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| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
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No definition available.
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