| **Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) (USD $)** | Mar. 31, 2014 | Dec. 31, 2013   |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|-----------------|
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                         |          |            |
| Less than 12 months Fair Value                                                                                                                                 | $ 840,000,000 | $ 1,516,000,000 |
| Less than 12 months Unrealized Losses                                                                                                                          | 30,000,000    | 74,000,000      |
| 12 months or more Fair Value                                                                                                                                   | 274,000,000   | 86,000,000      |
| 12 months or more Unrealized Losses                                                                                                                            | 19,000,000    | 5,000,000       |
| Total Fair Value                                                                                                                                               | 1,114,000,000 | 1,602,000,000   |
| Total Unrealized Losses                                                                                                                                        | 49,000,000    | 79,000,000      |
| Fixed Maturities                                                                                                                                               |          |            |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                         |          |            |
| Less than 12 months Fair Value                                                                                                                                 | 729,000,000   | 1,397,000,000   |
| Less than 12 months Unrealized Losses                                                                                                                          | 23,000,000    | 70,000,000      |
| 12 months or more Fair Value                                                                                                                                   | 274,000,000   | 86,000,000      |
| 12 months or more Unrealized Losses                                                                                                                            | 19,000,000    | 5,000,000       |
| Total Fair Value                                                                                                                                               | 1,003,000,000 | 1,483,000,000   |
| Total Unrealized Losses                                                                                                                                        | 42,000,000    | 75,000,000      |
| Fixed Maturities | States, Municipalities And Political Subdivisions                                                                                           |          |            |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                         |          |            |
| Less than 12 months Fair Value                                                                                                                                 | 289,000,000   | 490,000,000     |
| Less than 12 months Unrealized Losses                                                                                                                          | 7,000,000     | 18,000,000      |
| 12 months or more Fair Value                                                                                                                                   | 98,000,000    | 42,000,000      |
| 12 months or more Unrealized Losses                                                                                                                            | 5,000,000     | 3,000,000       |
| Total Fair Value                                                                                                                                               | 387,000,000   | 532,000,000     |
| Total Unrealized Losses                                                                                                                                        | 12,000,000    | 21,000,000      |
| Fixed Maturities | United States Government                                                                                                                    |          |            |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                         |          |            |
| Less than 12 months Fair Value                                                                                                                                 | 11,000,000    | 1,000,000       |
| 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                                                                                                                                               | 11,000,000    | 1,000,000       |
| Total Unrealized Losses                                                                                                                                        | 0             | 0               |
| Fixed Maturities | Government-Sponsored Enterprises                                                                                                            |          |            |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                         |          |            |
| Less than 12 months Fair Value                                                                                                                                 | 83,000,000    | 199,000,000     |
| Less than 12 months Unrealized Losses                                                                                                                          | 8,000,000     | 27,000,000      |
| 12 months or more Fair Value                                                                                                                                   | 96,000,000    | 1,000,000       |
| 12 months or more Unrealized Losses                                                                                                                            | 12,000,000    | 0               |
| Total Fair Value                                                                                                                                               | 179,000,000   | 200,000,000     |
| Total Unrealized Losses                                                                                                                                        | 20,000,000    | 27,000,000      |
| Fixed Maturities | Commercial Mortgage Backed Securities                                                                                                       |          |            |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                         |          |            |
| Less than 12 months Fair Value                                                                                                                                 | 55,000,000    |            |
| Less than 12 months Unrealized Losses                                                                                                                          | 1,000,000     |            |
| 12 months or more Fair Value                                                                                                                                   | 15,000,000    |            |
| 12 months or more Unrealized Losses                                                                                                                            | 1,000,000     |            |
| Total Fair Value                                                                                                                                               | 70,000,000    |            |
| Total Unrealized Losses                                                                                                                                        | 2,000,000     |            |
| Fixed Maturities | Corporate Securities                                                                                                                        |          |            |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                         |          |            |
| Less than 12 months Fair Value                                                                                                                                 | 291,000,000   | 572,000,000     |
| Less than 12 months Unrealized Losses                                                                                                                          | 7,000,000     | 20,000,000      |
| 12 months or more Fair Value                                                                                                                                   | 65,000,000    | 43,000,000      |
| 12 months or more Unrealized Losses                                                                                                                            | 1,000,000     | 2,000,000       |
| Total Fair Value                                                                                                                                               | 356,000,000   | 615,000,000     |
| Total Unrealized Losses                                                                                                                                        | 8,000,000     | 22,000,000      |
| Equity Securities                                                                                                                                              |          |            |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                         |          |            |
| Less than 12 months Fair Value                                                                                                                                 | 111,000,000   | 119,000,000     |
| Less than 12 months Unrealized Losses                                                                                                                          | 7,000,000     | 4,000,000       |
| 12 months or more Fair Value                                                                                                                                   | 0             | 0               |
| 12 months or more Unrealized Losses                                                                                                                            | 500,000,000   | 0               |
| Total Fair Value                                                                                                                                               | 111,000,000   | 119,000,000     |
| Total Unrealized Losses                                                                                                                                        | 7,000,000     | 4,000,000       |
| Equity Securities | Common Stock                                                                                                                               |          |            |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                         |          |            |
| Less than 12 months Fair Value                                                                                                                                 | 72,000,000    | 77,000,000      |
| Less than 12 months Unrealized Losses                                                                                                                          | 6,000,000     | 1,000,000       |
| 12 months or more Fair Value                                                                                                                                   | 0             |            |
| Total Fair Value                                                                                                                                               | 72,000,000    | 77,000,000      |
| Total Unrealized Losses                                                                                                                                        | 6,000,000     | 1,000,000       |
| Equity Securities | Preferred Stock                                                                                                                            |          |            |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                         |          |            |
| Less than 12 months Fair Value                                                                                                                                 | 39,000,000    | 42,000,000      |
| Less than 12 months Unrealized Losses                                                                                                                          | 1,000,000     | 3,000,000       |
| 12 months or more Fair Value                                                                                                                                   | 0             | 0               |
| 12 months or more Unrealized Losses                                                                                                                            |          | 0               |
| Total Fair Value                                                                                                                                               | 39,000,000    | 42,000,000      |
| Total Unrealized Losses                                                                                                                                        | $ 1,000,000   | $ 3,000,000     |

| 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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| 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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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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 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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