| **Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions** | Mar. 31, 2016 | Dec. 31, 2015 |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | $ 1,294       | $ 1,654       |
| Less than 12 months Unrealized Losses                                                                                                                                          | 89            | 119           |
| 12 months or more Fair Value                                                                                                                                                   | 178           | 284           |
| 12 months or more Unrealized Losses                                                                                                                                            | 24            | 37            |
| Total Fair Value                                                                                                                                                               | 1,472         | 1,938         |
| Total Unrealized Losses                                                                                                                                                        | 113           | 156           |
| Fixed Maturities                                                                                                                                                               |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 907           | 1,349         |
| Less than 12 months Unrealized Losses                                                                                                                                          | 44            | 68            |
| 12 months or more Fair Value                                                                                                                                                   | 178           | 284           |
| 12 months or more Unrealized Losses                                                                                                                                            | 24            | 37            |
| Total Fair Value                                                                                                                                                               | 1,085         | 1,633         |
| Total Unrealized Losses                                                                                                                                                        | 68            | 105           |
| Fixed Maturities | Corporate                                                                                                                                                   |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 801           | 1,099         |
| Less than 12 months Unrealized Losses                                                                                                                                          | 43            | 63            |
| 12 months or more Fair Value                                                                                                                                                   | 153           | 133           |
| 12 months or more Unrealized Losses                                                                                                                                            | 23            | 33            |
| Total Fair Value                                                                                                                                                               | 954           | 1,232         |
| Total Unrealized Losses                                                                                                                                                        | 66            | 96            |
| Fixed Maturities | States, municipalities and political subdivisions                                                                                                           |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 28            | 47            |
| Less than 12 months Unrealized Losses                                                                                                                                          | 0             | 1             |
| 12 months or more Fair Value                                                                                                                                                   | 4             | 22            |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 0             |
| Total Fair Value                                                                                                                                                               | 32            | 69            |
| Total Unrealized Losses                                                                                                                                                        | 0             | 1             |
| Fixed Maturities | Commercial mortgage-backed                                                                                                                                  |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 49            | 103           |
| Less than 12 months Unrealized Losses                                                                                                                                          | 1             | 2             |
| 12 months or more Fair Value                                                                                                                                                   | 2             | 2             |
| 12 months or more Unrealized Losses                                                                                                                                            | 1             | 0             |
| Total Fair Value                                                                                                                                                               | 51            | 105           |
| Total Unrealized Losses                                                                                                                                                        | 2             | 2             |
| Fixed Maturities | Government-sponsored enterprises                                                                                                                            |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 29            | 100           |
| Less than 12 months Unrealized Losses                                                                                                                                          | 0             | 2             |
| 12 months or more Fair Value                                                                                                                                                   | 19            | 127           |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 4             |
| Total Fair Value                                                                                                                                                               | 48            | 227           |
| Total Unrealized Losses                                                                                                                                                        | 0             | 6             |
| Equity Securities                                                                                                                                                              |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 387           | 305           |
| Less than 12 months Unrealized Losses                                                                                                                                          | 45            | 51            |
| 12 months or more Fair Value                                                                                                                                                   | 0             | 0             |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 0             |
| Total Fair Value                                                                                                                                                               | 387           | 305           |
| Total Unrealized Losses                                                                                                                                                        | 45            | 51            |
| Equity Securities | Common equities                                                                                                                                            |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 345           | 270           |
| Less than 12 months Unrealized Losses                                                                                                                                          | 44            | 51            |
| 12 months or more Fair Value                                                                                                                                                   | 0             | 0             |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 0             |
| Total Fair Value                                                                                                                                                               | 345           | 270           |
| Total Unrealized Losses                                                                                                                                                        | 44            | 51            |
| Equity Securities | Nonredeemable preferred equities                                                                                                                           |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 42            | 35            |
| Less than 12 months Unrealized Losses                                                                                                                                          | 1             | 0             |
| 12 months or more Fair Value                                                                                                                                                   | 0             | 0             |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 0             |
| Total Fair Value                                                                                                                                                               | 42            | 35            |
| Total Unrealized Losses                                                                                                                                                        | $ 1           | $ 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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+ References
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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27290-111563Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27337-111563
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| X                                                                                                                                                         |
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