| **Investments - Additional Information (Detail)**                                                                       | 3 Months Ended      | 12 Months Ended |
|-------------------------------------------------------------------------------------------------------------------------|---------------------|-----------------|
||
| **Gain (Loss) on Securities**                                                                                           |                |            |   |
| Debt Securities, Available-for-sale, Allowance for Credit Loss | security                                               | 0                   |            |   |
| Equity securities, at fair value                                                                                        | $ 6,225,000,000     |            | $ 7,752,000,000 |
| Largest Single Common Stock Holding                                                                                     | $ 395,000,000       |            |   |
| Largest equity holdings to total equity portfolio                                                                       | 6.60%               |            |   |
| Largest equity holdings to total portfolio                                                                              | 2.30%               |            |   |
| Fixed maturity investments, Percentage of amortized costs                                                               | 70.00%              |            | 70.00% |
| Fixed maturities                                                                                                        |                |            |   |
| **Gain (Loss) on Securities**                                                                                           |                |            |   |
| Writedown of Impaired Securities Intended to be Sold                                                                    | $ (77,000,000)      |            |   |
| Number Of Impaired Securities Intended to be Sold | security                                                            | 12                  |            |   |
| Number Of Securities Other Than Temporary Impairment Losses1 | security                                                 |                | 0               |   |
| Provision for Other Credit Losses                                                                                       | $ 0                 | $ 0             |   |
| Number Of Securities Continuous Unrealized Loss Position | security                                                     | 605                 |            |   |
| Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss 3                                    | $ 195,000,000       |            | $ 7,000,000 |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months | security                             |                |            | 38 |
| Unrealized losses 12 months or more (equity investment less than $500,000)                                              | $ (6,000,000)       |            | $ (3,000,000) |
| Number Of Securities Continuous Unrealized Loss Position and Below 70 Percent of Amortized Cost                         | 18                  |            |   |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost |                |            | 0 |
| Common equities | Equity securities                                                                                     |                |            |   |
| **Gain (Loss) on Securities**                                                                                           |                |            |   |
| Equity securities, at fair value                                                                                        | $ 6,017,000,000.000 |            | $ 7,518,000,000 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized loss on securities classified as available-for-sale that have been in a loss position for twelve months or longer.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized loss on securities classified as available-for-sale.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fixed maturity investments, Percentage of amortized costs.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Largest Single Common Stock Holding
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No definition available.
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| X                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
largest equity holdings to total equity portfolio
+ References
No definition available.
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| X                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Largest equity holdings to total portfolio
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of impaired available-for-sale debt securities intended to be sold and written down to fair value
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number Of Securities Continuous Unrealized Loss Position
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number Of Securities Continuous Unrealized Loss Position and Below 70 Percent of Amortized Cost
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No definition available.
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| X                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number Of Securities Other Than Temporary Impairment Losses
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of impaired available-for-sale debt securities not intended to be sold with an allowance for credit losses due to a credit loss
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Writedown of impaired available-for-sale debt securities to fair value due to intent to sell
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of equity securities categorized neither as held-to-maturity nor as trading.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of expense related to credit loss from transactions other than loan and lease transactions.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
No definition available.
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| X                                                                                                                                               |
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| X                                                                                                                                                 |
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| X                                                                                                                                                 |
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