| **Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions** | Sep. 30, 2015 | Dec. 31, 2014 |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | $ 1,651       | $ 393         |
| Less than 12 months Unrealized Losses                                                                                                                                          | 104           | 11            |
| 12 months or more Fair Value                                                                                                                                                   | 277           | 446           |
| 12 months or more Unrealized Losses                                                                                                                                            | 10            | 11            |
| Total Fair Value                                                                                                                                                               | 1,928         | 839           |
| Total Unrealized Losses                                                                                                                                                        | 114           | 22            |
| Fixed Maturities                                                                                                                                                               |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 1,097         | 292           |
| Less than 12 months Unrealized Losses                                                                                                                                          | 47            | 8             |
| 12 months or more Fair Value                                                                                                                                                   | 277           | 429           |
| 12 months or more Unrealized Losses                                                                                                                                            | 10            | 10            |
| Total Fair Value                                                                                                                                                               | 1,374         | 721           |
| Total Unrealized Losses                                                                                                                                                        | 57            | 18            |
| Fixed Maturities | Corporate                                                                                                                                                   |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 729           | 261           |
| Less than 12 months Unrealized Losses                                                                                                                                          | 42            | 8             |
| 12 months or more Fair Value                                                                                                                                                   | 85            | 90            |
| 12 months or more Unrealized Losses                                                                                                                                            | 7             | 3             |
| Total Fair Value                                                                                                                                                               | 814           | 351           |
| Total Unrealized Losses                                                                                                                                                        | 49            | 11            |
| Fixed Maturities | States, municipalities and political subdivisions                                                                                                           |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 227           | 17            |
| Less than 12 months Unrealized Losses                                                                                                                                          | 3             | 0             |
| 12 months or more Fair Value                                                                                                                                                   | 51            | 135           |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 2             |
| Total Fair Value                                                                                                                                                               | 278           | 152           |
| Total Unrealized Losses                                                                                                                                                        | 3             | 2             |
| Fixed Maturities | Commercial mortgage-backed                                                                                                                                  |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 57            | 3             |
| Less than 12 months Unrealized Losses                                                                                                                                          | 1             | 0             |
| 12 months or more Fair Value                                                                                                                                                   | 2             | 23            |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 0             |
| Total Fair Value                                                                                                                                                               | 59            | 26            |
| Total Unrealized Losses                                                                                                                                                        | 1             | 0             |
| Fixed Maturities | Government-sponsored enterprises                                                                                                                            |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 74            | 11            |
| Less than 12 months Unrealized Losses                                                                                                                                          | 1             | 0             |
| 12 months or more Fair Value                                                                                                                                                   | 139           | 181           |
| 12 months or more Unrealized Losses                                                                                                                                            | 3             | 5             |
| Total Fair Value                                                                                                                                                               | 213           | 192           |
| Total Unrealized Losses                                                                                                                                                        | 4             | 5             |
| Fixed Maturities | Foreign government                                                                                                                                          |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 10            |          |
| Less than 12 months Unrealized Losses                                                                                                                                          | 0             |          |
| 12 months or more Fair Value                                                                                                                                                   | 0             |          |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             |          |
| Total Fair Value                                                                                                                                                               | 10            |          |
| Total Unrealized Losses                                                                                                                                                        | 0             |          |
| Equity Securities                                                                                                                                                              |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 554           | 101           |
| Less than 12 months Unrealized Losses                                                                                                                                          | 57            | 3             |
| 12 months or more Fair Value                                                                                                                                                   | 0             | 17            |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 1             |
| Total Fair Value                                                                                                                                                               | 554           | 118           |
| Total Unrealized Losses                                                                                                                                                        | 57            | 4             |
| Equity Securities | Common equities                                                                                                                                            |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 511           | 85            |
| Less than 12 months Unrealized Losses                                                                                                                                          | 56            | 3             |
| 12 months or more Fair Value                                                                                                                                                   | 0             | 0             |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 0             |
| Total Fair Value                                                                                                                                                               | 511           | 85            |
| Total Unrealized Losses                                                                                                                                                        | 56            | 3             |
| Equity Securities | Nonredeemable preferred equities                                                                                                                           |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 43            | 16            |
| Less than 12 months Unrealized Losses                                                                                                                                          | 1             | 0             |
| 12 months or more Fair Value                                                                                                                                                   | 0             | 17            |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 1             |
| Total Fair Value                                                                                                                                                               | 43            | 33            |
| Total Unrealized Losses                                                                                                                                                        | $ 1           | $ 1           |

| 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                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Investments, Unrealized Loss Position [Line Items]
+ 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
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=27724398&loc=d3e27290-111563
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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
Reference 1: 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=27724398&loc=d3e27337-111563Reference 2: 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=27724398&loc=d3e27290-111563
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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=27724398&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=27724398&loc=d3e27337-111563
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| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                        |
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| X                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                           |
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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