| **Investments - Realized Investment Gains and Losses and Change in Unrealized Investment Gains and Losses and Other Items (Detail) - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Investment income:**                                                                                                                                       |           |           |   |   |
| Interest                                                                                                                                                     | $ 129          | $ 121          | $ 376 | $ 356 |
| Dividends                                                                                                                                                    | 66             | 61             | 203 | 179 |
| Other                                                                                                                                                        | 3              | 1              | 6 | 4 |
| Total                                                                                                                                                        | 198            | 183            | 585 | 539 |
| Less investment expenses                                                                                                                                     | 5              | 4              | 12 | 11 |
| Total                                                                                                                                                        | 193            | 179            | 573 | 528 |
| Gain (Loss) on Sale of Investments                                                                                                                           |           |           | (2,513) | 933 |
| **Investment gains and losses, net:**                                                                                                                        |           |           |   |   |
| Investment gains and losses, net                                                                                                                             | (674)          | (70)           | (2,494) | 954 |
| Equity securities                                                                                                                                            |           |           |   |   |
| **Investment income:**                                                                                                                                       |           |           |   |   |
| Gain (Loss) on Sale of Investments                                                                                                                           | 16             | (1)            | 34 | 6 |
| Unrealized Gain (Loss) on Investments                                                                                                                        | (705)          | (104)          | (2,568) | 869 |
| **Investment gains and losses, net:**                                                                                                                        |           |           |   |   |
| Other                                                                                                                                                        | 15             | 27             | 37 | 59 |
| Investment gains and losses, net                                                                                                                             | (689)          | (105)          | (2,534) | 875 |
| Fixed maturities                                                                                                                                             |           |           |   |   |
| **Investment gains and losses, net:**                                                                                                                        |           |           |   |   |
| Gross realized gains                                                                                                                                         | 0              | 10             | 6 | 24 |
| Available-for-sale Securities, Gross Realized Losses                                                                                                         | 0              | (1)            | (3) | (3) |
| Writedown of Impaired Securities Intended to be Sold                                                                                                         | 0              | (1)            | 0 | (1) |
| Investment gains and losses, net                                                                                                                             | $ 0            | $ 8            | $ 3 | $ 20 |

| X                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Income derived from those other than interest and dividends.
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| - Definition
This item represents realized investment gains (losses) included in earnings for the period excluding those relating to other than temporary impairment losses on investments in debt securities. Includes realized investment gains and losses generated from the sale of debt securities, equity securities, investments in joint ventures and limited partnerships and from other than temporary impairments on investments other than debt securities. Also includes realized investment gains and losses are generated from prepayment premiums, recoveries of principal on previously impaired securities, provisions for losses on commercial mortgage and other loans, and fair value changes on embedded derivatives and derivatives that do not qualify for hedge accounting treatment, except those derivatives used in the Company's capacity as a broker or dealer.
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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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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
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| - Definition
Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).
+ References
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| - Definition
Amount of realized 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
The net realized gain (loss) on investments sold during the period, not including gains (losses) on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity, which, for cash flow reporting, is a component of proceeds from investing activities.
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| - Definition
Amount, before investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.
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| - Definition
Amount of dividend income on nonoperating securities.
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| - Definition
Amount before accretion (amortization) of purchase discount (premium) of interest income on nonoperating securities.
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| - Definition
Amount of expenses related to the generation of investment income.
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| - Definition
Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.
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Amount of realized gain (loss) on investment.
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| - Definition
Amount of unrealized gain (loss) on investment.
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