| **Investments - Realized Investment Gains and Losses and Change in Unrealized Investment Gains and Losses and Other Items (Detail) (USD $)In Millions, unless otherwise specified** | 3 Months Ended | 9 Months Ended |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Investment income summary:**                                                                                                                                                      |           |           |   |   |
| Interest on fixed maturities                                                                                                                                                        | $ 104          | $ 105          | $ 309 | $ 317 |
| Dividends on equity securities                                                                                                                                                      | 30             | 28             | 87 | 81 |
| Other investment income                                                                                                                                                             | 1              | 1              | 2 | 3 |
| Total                                                                                                                                                                               | 135            | 134            | 398 | 401 |
| Less investment expenses                                                                                                                                                            | 2              | 2              | 6 | 6 |
| Total                                                                                                                                                                               | 133            | 132            | 392 | 395 |
| **Realized investment gains and losses summary:**                                                                                                                                   |           |           |   |   |
| Other                                                                                                                                                                               | (2)            | 0              | (7) | 5 |
| Realized gains on investments                                                                                                                                                       | 22             | 10             | 77 | 29 |
| **Change in unrealized gains and losses summary:**                                                                                                                                  |           |           |   |   |
| Unrealized gains (losses) on investments available-for-sale                                                                                                                         | 40             | 164            | 113 | 278 |
| Adjustment to deferred acquisition costs and life policy reserves                                                                                                                   | 9              | (6)            | 38 | (21) |
| Amortization of pension actuarial loss and prior service cost                                                                                                                       | 2              | 2              | 7 | 5 |
| Other                                                                                                                                                                               | (2)            | 0              | (7) | 5 |
| Income taxes on above                                                                                                                                                               | (25)           | (88)           | 74 | 147 |
| Other comprehensive income (loss), net of tax                                                                                                                                       | 46             | 161            | 138 | 270 |
| Fixed Maturities                                                                                                                                                                    |           |           |   |   |
| **Realized investment gains and losses summary:**                                                                                                                                   |           |           |   |   |
| Gross realized gains                                                                                                                                                                | 5              | 14             | 9 | 30 |
| Gross realized losses                                                                                                                                                               | 0              | 0              | 0 | 0 |
| Other-than-temporary impairments                                                                                                                                                    | 0              | 0              | (2) | 0 |
| **Change in unrealized gains and losses summary:**                                                                                                                                  |           |           |   |   |
| Unrealized gains (losses) on investments available-for-sale                                                                                                                         | (28)           | 110            | (335) | 188 |
| Equity Securities                                                                                                                                                                   |           |           |   |   |
| **Realized investment gains and losses summary:**                                                                                                                                   |           |           |   |   |
| Gross realized gains                                                                                                                                                                | 15             | 1              | 64 | 30 |
| Gross realized losses                                                                                                                                                               | 0              | 0              | 0 | (1) |
| Other-than-temporary impairments                                                                                                                                                    | 0              | (2)            | 0 | (32) |
| Securities with embedded derivatives                                                                                                                                                | 0              | (4)            | 1 | 1 |
| Other                                                                                                                                                                               | 2              | 1              | 5 | 1 |
| Realized gains on investments                                                                                                                                                       | 22             | 10             |   |   |
| **Change in unrealized gains and losses summary:**                                                                                                                                  |           |           |   |   |
| Unrealized gains (losses) on investments available-for-sale                                                                                                                         | 90             | 143            | 509 | 240 |
| Other                                                                                                                                                                               | $ 2            | $ 1            | $ 5 | $ 1 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Income derived from investments in debt and equity securities, cash and cash equivalents, and other long-term investments. Interest income represents earnings which reflect the time value of money or transactions in which the payments are for the use or forbearance of money. Dividend income represents a distribution of earnings to shareholders by investee companies. Other income represents all other investment income not including interest and dividends.
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Income derived from those other than interest and dividends.
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| X                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Other Comprehensive Income, Unrealized gains losses on deferred acquisition costs , Period Increase (Decrease)
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| - Definition
Other Comprehensive Income (Loss), Amortization of Pension Actuarial Loss And Prior Service Cost
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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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| X                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Other Than Temporary Impairment Losses
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.
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| - Definition
This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.
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| - Definition
Net Increase or Decrease in the fair value of the embedded derivative or group of embedded derivatives included in earnings in the period.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax and reclassification adjustments of other comprehensive income (loss).
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of tax expense (benefit) allocated to other comprehensive income (loss).
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| - Definition
Amount before tax and reclassification adjustments of unrealized holding gain (loss) on available-for-sale securities.
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| - Definition
The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.
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