| **Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) (USD $)In Millions, unless otherwise specified** | Sep. 30, 2013 | Dec. 31, 2012 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | $ 1,237       | $ 223         |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 64            | 12            |
| 12 months or more Fair Value                                                                                                                                                                          | 43            | 17            |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 3             | 1             |
| Total Fair Value                                                                                                                                                                                      | 1,280         | 240           |
| Total Unrealized Losses                                                                                                                                                                               | 67            | 13            |
| Fixed Maturities                                                                                                                                                                                      |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 1,159         | 112           |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 61            | 2             |
| 12 months or more Fair Value                                                                                                                                                                          | 43            | 17            |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 3             | 1             |
| Total Fair Value                                                                                                                                                                                      | 1,202         | 129           |
| Total Unrealized Losses                                                                                                                                                                               | 64            | 3             |
| Fixed Maturities | States, Municipalities And Political Subdivisions                                                                                                                                  |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 371           | 53            |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 17            | 1             |
| 12 months or more Fair Value                                                                                                                                                                          | 8             | 0             |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 0             | 0             |
| Total Fair Value                                                                                                                                                                                      | 379           | 53            |
| Total Unrealized Losses                                                                                                                                                                               | 17            | 1             |
| Fixed Maturities | Foreign Government Debt Securities [Member]                                                                                                                                        |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 9             |          |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 0             |          |
| 12 months or more Fair Value                                                                                                                                                                          | 0             |          |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 0             |          |
| Total Fair Value                                                                                                                                                                                      | 9             |          |
| Total Unrealized Losses                                                                                                                                                                               | 0             |          |
| Fixed Maturities | United States Government                                                                                                                                                           |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 1             |          |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 0             |          |
| 12 months or more Fair Value                                                                                                                                                                          | 0             |          |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 0             |          |
| Total Fair Value                                                                                                                                                                                      | 1             |          |
| Total Unrealized Losses                                                                                                                                                                               | 0             |          |
| Fixed Maturities | Government-Sponsored Enterprises                                                                                                                                                   |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 190           | 1             |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 19            | 0             |
| 12 months or more Fair Value                                                                                                                                                                          | 0             | 0             |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 0             | 0             |
| Total Fair Value                                                                                                                                                                                      | 190           | 1             |
| Total Unrealized Losses                                                                                                                                                                               | 19            | 0             |
| Fixed Maturities | Commercial Mortgage Backed Securities                                                                                                                                              |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 62            |          |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 3             |          |
| 12 months or more Fair Value                                                                                                                                                                          | 0             |          |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 0             |          |
| Total Fair Value                                                                                                                                                                                      | 62            |          |
| Total Unrealized Losses                                                                                                                                                                               | 3             |          |
| Fixed Maturities | Corporate Securities                                                                                                                                                               |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 526           | 58            |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 22            | 1             |
| 12 months or more Fair Value                                                                                                                                                                          | 35            | 17            |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 3             | 1             |
| Total Fair Value                                                                                                                                                                                      | 561           | 75            |
| Total Unrealized Losses                                                                                                                                                                               | 25            | 2             |
| Equity Securities                                                                                                                                                                                     |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 78            | 111           |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 3             | 10            |
| 12 months or more Fair Value                                                                                                                                                                          | 0             | 0             |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 0             | 0             |
| Total Fair Value                                                                                                                                                                                      | 78            | 111           |
| Total Unrealized Losses                                                                                                                                                                               | 3             | 10            |
| Equity Securities | Common Stock                                                                                                                                                                      |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 74            | 107           |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 3             | 9             |
| 12 months or more Fair Value                                                                                                                                                                          | 0             |          |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 0             |          |
| Total Fair Value                                                                                                                                                                                      | 74            | 107           |
| Total Unrealized Losses                                                                                                                                                                               | 3             | 9             |
| Equity Securities | Preferred Stock                                                                                                                                                                   |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 4             | 4             |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 0             | 1             |
| 12 months or more Fair Value                                                                                                                                                                          | 0             | 0             |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 0             | 0             |
| Total Fair Value                                                                                                                                                                                      | 4             | 4             |
| Total Unrealized Losses                                                                                                                                                                               | $ 0           | $ 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                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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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 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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