| **Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions** | Jun. 30, 2017 | Dec. 31, 2016 |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | $ 1,559       | $ 2,079       |
| Less than 12 months Unrealized Losses                                                                                                                                          | 45            | 71            |
| 12 months or more Fair Value                                                                                                                                                   | 141           | 191           |
| 12 months or more Unrealized Losses                                                                                                                                            | 6             | 11            |
| Total Fair Value                                                                                                                                                               | 1,700         | 2,270         |
| Total Unrealized Losses                                                                                                                                                        | 51            | 82            |
| Fixed Maturities                                                                                                                                                               |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 1,205         | 1,972         |
| Less than 12 months Unrealized Losses                                                                                                                                          | 29            | 62            |
| 12 months or more Fair Value                                                                                                                                                   | 141           | 191           |
| 12 months or more Unrealized Losses                                                                                                                                            | 6             | 11            |
| Total Fair Value                                                                                                                                                               | 1,346         | 2,163         |
| Total Unrealized Losses                                                                                                                                                        | 35            | 73            |
| Fixed Maturities | Corporate                                                                                                                                                   |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 290           | 733           |
| Less than 12 months Unrealized Losses                                                                                                                                          | 8             | 15            |
| 12 months or more Fair Value                                                                                                                                                   | 136           | 189           |
| 12 months or more Unrealized Losses                                                                                                                                            | 6             | 11            |
| Total Fair Value                                                                                                                                                               | 426           | 922           |
| Total Unrealized Losses                                                                                                                                                        | 14            | 26            |
| Fixed Maturities | States, municipalities and political subdivisions                                                                                                           |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 665           | 989           |
| Less than 12 months Unrealized Losses                                                                                                                                          | 17            | 42            |
| 12 months or more Fair Value                                                                                                                                                   | 2             | 0             |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 0             |
| Total Fair Value                                                                                                                                                               | 667           | 989           |
| Total Unrealized Losses                                                                                                                                                        | 17            | 42            |
| Fixed Maturities | Commercial mortgage-backed                                                                                                                                  |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 53            | 89            |
| Less than 12 months Unrealized Losses                                                                                                                                          | 1             | 2             |
| 12 months or more Fair Value                                                                                                                                                   | 3             | 2             |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 0             |
| Total Fair Value                                                                                                                                                               | 56            | 91            |
| Total Unrealized Losses                                                                                                                                                        | 1             | 2             |
| Fixed Maturities | Government-sponsored enterprises                                                                                                                            |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 190           | 155           |
| Less than 12 months Unrealized Losses                                                                                                                                          | 3             | 3             |
| 12 months or more Fair Value                                                                                                                                                   | 0             | 0             |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 0             |
| Total Fair Value                                                                                                                                                               | 190           | 155           |
| Total Unrealized Losses                                                                                                                                                        | 3             | 3             |
| Fixed Maturities | United States government                                                                                                                                    |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 7             | 6             |
| Less than 12 months Unrealized Losses                                                                                                                                          | 0             | 0             |
| 12 months or more Fair Value                                                                                                                                                   | 0             | 0             |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 0             |
| Total Fair Value                                                                                                                                                               | 7             | 6             |
| Total Unrealized Losses                                                                                                                                                        | 0             | 0             |
| Equity Securities                                                                                                                                                              |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 354           | 107           |
| Less than 12 months Unrealized Losses                                                                                                                                          | 16            | 9             |
| 12 months or more Fair Value                                                                                                                                                   | 0             | 0             |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 0             |
| Total Fair Value                                                                                                                                                               | 354           | 107           |
| Total Unrealized Losses                                                                                                                                                        | 16            | 9             |
| Equity Securities | Common equities                                                                                                                                            |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 | 354           | 103           |
| Less than 12 months Unrealized Losses                                                                                                                                          | 16            | 9             |
| 12 months or more Fair Value                                                                                                                                                   | 0             | 0             |
| 12 months or more Unrealized Losses                                                                                                                                            | 0             | 0             |
| Total Fair Value                                                                                                                                                               | 354           | 103           |
| Total Unrealized Losses                                                                                                                                                        | $ 16          | 9             |
| Equity Securities | Nonredeemable preferred equities                                                                                                                           |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                 |          | 4             |
| Less than 12 months Unrealized Losses                                                                                                                                          |          | 0             |
| 12 months or more Fair Value                                                                                                                                                   |          | 0             |
| 12 months or more Unrealized Losses                                                                                                                                            |          | 0             |
| Total Fair Value                                                                                                                                                               |          | 4             |
| 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                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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=73527792&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=73527792&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=73527792&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=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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| X                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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