| **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** | Mar. 31, 2013 | Dec. 31, 2012 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | $ 419         | $ 223         |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 5             | 12            |
| 12 months or more Fair Value                                                                                                                                                                          | 13            | 17            |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 1             | 1             |
| Total Fair Value                                                                                                                                                                                      | 432           | 240           |
| Total Unrealized Losses                                                                                                                                                                               | 6             | 13            |
| Fixed Maturities                                                                                                                                                                                      |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 344           | 112           |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 3             | 2             |
| 12 months or more Fair Value                                                                                                                                                                          | 13            | 17            |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 1             | 1             |
| Total Fair Value                                                                                                                                                                                      | 357           | 129           |
| Total Unrealized Losses                                                                                                                                                                               | 4             | 3             |
| Fixed Maturities | States, Municipalities And Political Subdivisions                                                                                                                                  |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 103           | 53            |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 1             | 1             |
| 12 months or more Fair Value                                                                                                                                                                          | 0             | 0             |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 0             | 0             |
| Total Fair Value                                                                                                                                                                                      | 103           | 53            |
| Total Unrealized Losses                                                                                                                                                                               | 1             | 1             |
| Fixed Maturities | Government-Sponsored Enterprises                                                                                                                                                   |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 107           | 1             |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 1             | 0             |
| 12 months or more Fair Value                                                                                                                                                                          | 0             | 0             |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 0             | 0             |
| Total Fair Value                                                                                                                                                                                      | 107           | 1             |
| Total Unrealized Losses                                                                                                                                                                               | 1             | 0             |
| Fixed Maturities | Commercial Mortgage Backed Securities                                                                                                                                              |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 16            |          |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 0             |          |
| 12 months or more Fair Value                                                                                                                                                                          | 0             |          |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 0             |          |
| Total Fair Value                                                                                                                                                                                      | 16            |          |
| Total Unrealized Losses                                                                                                                                                                               | 0             |          |
| Fixed Maturities | Corporate Securities                                                                                                                                                               |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 118           | 58            |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 1             | 1             |
| 12 months or more Fair Value                                                                                                                                                                          | 13            | 17            |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 1             | 1             |
| Total Fair Value                                                                                                                                                                                      | 131           | 75            |
| Total Unrealized Losses                                                                                                                                                                               | 2             | 2             |
| Equity Securities                                                                                                                                                                                     |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 75            | 111           |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 2             | 10            |
| 12 months or more Fair Value                                                                                                                                                                          | 0             | 0             |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 0             | 0             |
| Total Fair Value                                                                                                                                                                                      | 75            | 111           |
| Total Unrealized Losses                                                                                                                                                                               | 2             | 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                                                                                                                                                                 | 2             | 9             |
| 12 months or more Fair Value                                                                                                                                                                          | 0             |          |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 0             |          |
| Total Fair Value                                                                                                                                                                                      | 74            | 107           |
| Total Unrealized Losses                                                                                                                                                                               | 2             | 9             |
| Equity Securities | Preferred Stock                                                                                                                                                                   |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 1             | 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                                                                                                                                                                                      | 1             | 4             |
| Total Unrealized Losses                                                                                                                                                                               | $ 0           | $ 1           |

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Amount of the excess of amortized cost basis over fair value of securities that have been in a loss position for twelve months or longer for securities which are categorized neither as held-to-maturity nor trading securities.
+ References
No definition available.
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| - Definition
Amount of the excess of amortized cost basis over fair value of securities in a loss position and categorized neither as held-to-maturity nor trading securities.
+ References
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| - 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.
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| - Definition
Amount of the excess of amortized cost basis over fair value of securities that have been in a loss position for less than twelve months for securities categorized neither as held-to-maturity nor trading securities.
+ References
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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 less than twelve months.
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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
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