| **Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions** | Sep. 30, 2016 | Dec. 31, 2015 |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | $ 522         | $ 1,654       |
| Less than 12 months Unrealized Losses                                                                                                                                          | 11            | 119           |
| 12 months or more Fair Value                                                                                                                                                   | 406           | 284           |
| 12 months or more Unrealized Losses                                                                                                                                            | 65            | 37            |
| Total Fair Value                                                                                                                                                               | 928           | 1,938         |
| Total Unrealized Losses                                                                                                                                                        | 76            | 156           |
| Fixed Maturities                                                                                                                                                               |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 345           | 1,349         |
| Less than 12 months Unrealized Losses                                                                                                                                          | 5             | 68            |
| 12 months or more Fair Value                                                                                                                                                   | 203           | 284           |
| 12 months or more Unrealized Losses                                                                                                                                            | 10            | 37            |
| Total Fair Value                                                                                                                                                               | 548           | 1,633         |
| Total Unrealized Losses                                                                                                                                                        | 15            | 105           |
| Fixed Maturities | Corporate                                                                                                                                                   |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 166           | 1,099         |
| Less than 12 months Unrealized Losses                                                                                                                                          | 4             | 63            |
| 12 months or more Fair Value                                                                                                                                                   | 200           | 133           |
| 12 months or more Unrealized Losses                                                                                                                                            | 10            | 33            |
| Total Fair Value                                                                                                                                                               | 366           | 1,232         |
| Total Unrealized Losses                                                                                                                                                        | 14            | 96            |
| Fixed Maturities | States, municipalities and political subdivisions                                                                                                           |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 128           | 47            |
| Less than 12 months Unrealized Losses                                                                                                                                          | 1             | 1             |
| 12 months or more Fair Value                                                                                                                                                   | 0             | 22            |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 0             |
| Total Fair Value                                                                                                                                                               | 128           | 69            |
| Total Unrealized Losses                                                                                                                                                        | 1             | 1             |
| Fixed Maturities | Commercial mortgage-backed                                                                                                                                  |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 8             | 103           |
| Less than 12 months Unrealized Losses                                                                                                                                          | 0             | 2             |
| 12 months or more Fair Value                                                                                                                                                   | 3             | 2             |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 0             |
| Total Fair Value                                                                                                                                                               | 11            | 105           |
| Total Unrealized Losses                                                                                                                                                        | 0             | 2             |
| Fixed Maturities | Government-sponsored enterprises                                                                                                                            |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 43            | 100           |
| Less than 12 months Unrealized Losses                                                                                                                                          | 0             | 2             |
| 12 months or more Fair Value                                                                                                                                                   | 0             | 127           |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 4             |
| Total Fair Value                                                                                                                                                               | 43            | 227           |
| Total Unrealized Losses                                                                                                                                                        | 0             | 6             |
| Equity Securities                                                                                                                                                              |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 |          | 305           |
| Less than 12 months Unrealized Losses                                                                                                                                          |          | 51            |
| 12 months or more Fair Value                                                                                                                                                   |          | 0             |
| 12 months or more Unrealized Losses                                                                                                                                            | 55            | 0             |
| Total Fair Value                                                                                                                                                               |          | 305           |
| Total Unrealized Losses                                                                                                                                                        |          | 51            |
| Equity Securities | Common equities                                                                                                                                            |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 177           | 270           |
| Less than 12 months Unrealized Losses                                                                                                                                          | 6             | 51            |
| 12 months or more Fair Value                                                                                                                                                   | 203           | 0             |
| 12 months or more Unrealized Losses                                                                                                                                            | 55            | 0             |
| Total Fair Value                                                                                                                                                               | 380           | 270           |
| Total Unrealized Losses                                                                                                                                                        | $ 61          | 51            |
| Equity Securities | Nonredeemable preferred equities                                                                                                                           |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 |          | 35            |
| Less than 12 months Unrealized Losses                                                                                                                                          |          | 0             |
| 12 months or more Fair Value                                                                                                                                                   |          | 0             |
| 12 months or more Unrealized Losses                                                                                                                                            |          | 0             |
| Total Fair Value                                                                                                                                                               |          | 35            |
| Total Unrealized Losses                                                                                                                                                        |          | $ 0           |

| 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
Amount of unrealized loss on investments in available-for-sale securities that have been in a loss position for less than twelve months.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                              |
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No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for twelve months or longer.
+ References
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