| **Investments - Additional Information (Details) $ in Millions**                                | 3 Months Ended | 12 Months Ended |
|-------------------------------------------------------------------------------------------------|----------------|-----------------|
||
| **Gain (Loss) on Securities**                                                                   |           |            |   |
| Equity Securities at Fair Value                                                                 | $ 9,967        |            | $ 9,841 |
| Largest Single Common Stock Holding                                                             | $ 758          |            | $ 597 |
| Largest equity holdings to total equity portfolio                                               | 7.90%          |            | 6.30% |
| Largest equity holdings to total portfolio                                                      | 3.30%          |            | 2.70% |
| Debt Securities, Available-for-sale, Allowance for Credit Loss                                  | $ 1            |            | $ 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                                    | 0              | 1               |   |
| Number Of Securities Continuous Unrealized Loss Position | security                             | 2,900          |            | 3,272 |
| Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss 3            | $ 776          |            | $ 908 |
| Number Of Securities Continuous Unrealized Loss Position and Below 70 Percent of Amortized Cost | 26             |            | 49 |
| Fixed maturity investments, Percentage of amortized costs                                       | 70.00%         |            | 70.00% |

| X                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized loss on securities classified as available-for-sale.
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| - Definition
Changes in Allowance for Credit Losses During Period
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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                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
largest equity holdings to total equity portfolio
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| X                                                                                                                                                                                                                                                                      |
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| - Definition
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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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ References
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