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Investments (Narrative) (Detail) (USD $) In Millions, unless otherwise specified | Sep. 30, 2012 Y | Dec. 31, 2011 Y
--- | --- | ---
Schedule of Investments [Line Items] |  |
Fair value of securities on deposit with various regulatory authorities | $ 10 | $ 11
Fair value of securities pledged as collateral | 885.0 | 1,900.0
Cash pledged as collateral under investment agreements | 126 | 224
Fair value | 1,025 | 2,387
Gross unrealized losses | 191 | 478
Gross unrealized losses, Investment portfolio | 359 | 849
Weighted average contractual maturity period in years for securities in an unrealized loss position | 21 | 21
Number of securities, unrealized loss position for continuous twelve-month period or longer | 193 | 290
12 months or longer, unrealized losses | 358 | 782
Rate of book value securities exceeded market value | 5.00% | 5.00%
Held-To-Maturity [Member] |  |
Schedule of Investments [Line Items] |  |
Unrecognized gross losses | 168 | 371
Unrecognized gross gains | 20 | 17
Asset-Backed Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value | 273 |
Gross unrealized losses | 99 |
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 | 44.00% |
Fair value of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors | 135 |
Percentage of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors | 49.00% |
Asset-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | 3rd Party Guarantors [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include the benefit of guarantees provided by financial guarantors | 27 |
Asset-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | MBIA Corp [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include the benefit of guarantees provided by financial guarantors | 148 |
Asset-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | Asset / Liability Products [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value | 57 |
Gross unrealized losses | 52 |
Mortgage-Backed Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value | 270 |
Gross unrealized losses | 52 |
Mortgage-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] |  |
Schedule of Investments [Line Items] |  |
Investments rated AAA | 18.00% |
Percentage of investments rated investment grade | 62.00% |
Fair value of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors | 197 |
Percentage of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors | 73.00% |
Mortgage-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | 3rd Party Guarantors [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include the benefit of guarantees provided by financial guarantors | 184 |
Mortgage-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | MBIA Corp [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include the benefit of guarantees provided by financial guarantors | 10 |
Corporate Obligations [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value | 163 |
Gross unrealized losses | 28 |
Corporate Obligations [Member] | Securities In An Unrealized Loss Position [Member] |  |
Schedule of Investments [Line Items] |  |
Percentage of investments rated investment grade | 84.00% |
Fair value of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors | 14 |
Percentage of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors | 9.00% |
Corporate Obligations [Member] | Securities In An Unrealized Loss Position [Member] | 3rd Party Guarantors [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include the benefit of guarantees provided by financial guarantors | $ 9 |
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 | 118 | 218

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