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Investments (Narrative) (Detail) (USD $) | Jun. 30, 2012 Year | Dec. 31, 2011 Year
--- | --- | ---
Schedule of Investments [Line Items] |  |
Fair value of securities on deposit with various regulatory authorities | $ 10,000,000 | $ 11,000,000
Fair value of securities pledged as collateral | 1,000,000,000 | 1,900,000,000
Cash pledged as collateral under investment agreements | 174,000,000 | 224,000,000
Fair value | 1,558,000,000 | 2,387,000,000
Gross unrealized losses | 226,000,000 | 478,000,000
Gross unrealized losses, Investment portfolio | 386,000,000 | 849,000,000
Weighted average contractual maturity period in years for securities in an unrealized loss position | 20 | 21
Number of securities, unrealized loss position for continuous twelve-month period or longer | 204 | 290
12 months or longer, unrealized losses | 381,000,000 | 782,000,000
Rate of book value securities exceeded market value | 5.00% | 5.00%
Held-To-Maturity [Member] |  |
Schedule of Investments [Line Items] |  |
Unrecognized gross losses | 160,000,000 | 371,000,000
Unrecognized gross gains | 5,000,000 | 17,000,000
Book Value Greater Than Market Value By More Than 5% [Member] |  |
Schedule of Investments [Line Items] |  |
Number of securities, unrealized loss position for continuous twelve-month period or longer | 131 | 218
Asset-Backed Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value | 288,000,000 |
Gross unrealized losses | 119,000,000 |
Asset-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] |  |
Schedule of Investments [Line Items] |  |
Investments rated AAA | 17.00% |
Percentage of investments rated investment grade | 53.00% |
Fair Value of Securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors | 134,000,000 |
Percentage of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors | 47.00% |
Asset-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | 3rd Party Guarantors [Member] |  |
Schedule of Investments [Line Items] |  |
Benefit of guarantees provided | 31,000,000 |
Asset-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | MBIA Corp [Member] |  |
Schedule of Investments [Line Items] |  |
Benefit of guarantees provided | 147,000,000 |
Asset-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | Asset / Liability Products [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value | 65,000,000 |
Gross unrealized losses | 57,000,000 |
Mortgage-Backed Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value | 284,000,000 |
Gross unrealized losses | 63,000,000 |
Mortgage-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] |  |
Schedule of Investments [Line Items] |  |
Investments rated AAA | 19.00% |
Percentage of investments rated investment grade | 61.00% |
Fair Value of Securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors | 209,000,000 |
Percentage of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors | 74.00% |
Mortgage-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | 3rd Party Guarantors [Member] |  |
Schedule of Investments [Line Items] |  |
Benefit of guarantees provided | 189,000,000 |
Mortgage-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | MBIA Corp [Member] |  |
Schedule of Investments [Line Items] |  |
Benefit of guarantees provided | 11,000,000 |
Corporate Obligations [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value | 305,000,000 |
Gross unrealized losses | 31,000,000 |
Corporate Obligations [Member] | Securities In An Unrealized Loss Position [Member] |  |
Schedule of Investments [Line Items] |  |
Investments rated AAA | 1.00% |
Percentage of investments rated investment grade | 92.00% |
Fair Value of Securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors | 13,000,000 |
Percentage of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors | 4.00% |
Corporate Obligations [Member] | Securities In An Unrealized Loss Position [Member] | 3rd Party Guarantors [Member] |  |
Schedule of Investments [Line Items] |  |
Benefit of guarantees provided | $ 9,000,000 |

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