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Investments (Narrative) (Details) (USD $) | 3 Months Ended | 9 Months Ended |  |  | 3 Months Ended | 9 Months Ended |  |  | 9 Months Ended |  | 9 Months Ended |  | 9 Months Ended |  |  | 9 Months Ended | 12 Months Ended
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Sep. 30, 2011 | Sep. 30, 2010 | Sep. 30, 2011 | Sep. 30, 2010 | Dec. 31, 2010 | Sep. 30, 2011 Asset-Backed Securities [Member] Asset / Liability Products [Member] | Sep. 30, 2011 Asset / Liability Products [Member] | Sep. 30, 2010 Asset / Liability Products [Member] | Sep. 30, 2011 Asset / Liability Products [Member] | Sep. 30, 2010 Asset / Liability Products [Member] | Sep. 30, 2011 National [Member] Corporate Obligations [Member] | Sep. 30, 2011 Mortgage-Backed Securities [Member] 3rd Party Guarantors [Member] | Sep. 30, 2011 Asset-Backed Securities [Member] 3rd Party Guarantors [Member] | Sep. 30, 2011 3rd Party Guarantors [Member] Corporate Obligations [Member] | Sep. 30, 2011 Book Value Greater Than Market Value By More Than 5% [Member] | Dec. 31, 2010 Book Value Greater Than Market Value By More Than 5% [Member] | Sep. 30, 2011 Asset-Backed Securities [Member] | Sep. 30, 2011 Asset-Backed Securities [Member] MBIA [Member] | Sep. 30, 2011 Asset-Backed Securities [Member] MBIA Corp [Member] | Sep. 30, 2011 Mortgage-Backed Securities [Member] | Sep. 30, 2011 Mortgage-Backed Securities [Member] MBIA [Member] | Sep. 30, 2011 Mortgage-Backed Securities [Member] MBIA Corp [Member] | Sep. 30, 2011 Corporate Obligations [Member] | Sep. 30, 2011 Corporate Obligations [Member] MBIA [Member] | Sep. 30, 2011 Corporate Obligations [Member] MBIA Corp [Member] | Sep. 30, 2011 Variable Interest Entity, Primary Beneficiary [Member] | Dec. 31, 2010 Variable Interest Entity, Primary Beneficiary [Member] | Sep. 30, 2011 Aggregate Unrealized Losses [Member] | Dec. 31, 2010 Aggregate Unrealized Losses [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value of securities on deposit with various regulatory authorities | $ 11,000,000 |  | $ 11,000,000 |  | $ 11,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value of securities pledged as collateral | 1,800,000,000 |  | 1,800,000,000 |  | 2,400,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash pledged as collateral under investment agreements | 378,000,000 |  | 378,000,000 |  | 113,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrecognized gross losses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 394,000,000 | 279,000,000 |  |
Gross unrealized losses, portfolio | 961,000,000 |  | 961,000,000 |  | 1,100,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross unrealized losses, available for sale securities | (567,000,000) |  | (567,000,000) |  | (804,000,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
12 months or longer, unrealized losses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 893,000,000 | 941,000,000
Rate of book value securities exceeded market value | 5.00% |  | 5.00% |  | 5.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of securities, unrealized loss position for continuous 12-month period or longer | 313 |  | 313 |  | 412 |  |  |  |  |  |  |  |  |  | 215 | 321 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average contractual maturity period of securities, unrealized loss position | 22 |  | 22 |  | 20 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate of security impaired at time of review |  |  | 20.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate of security impaired at time of review, fair value below amortized cost for consecutive 12-month period |  |  | 5.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of investments rated investment grade |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 60.00% |  |  |  | 62.00% |  |  |  |  |  |  |  |  |  |  |
Investment grade rated with AAA |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 21.00% |  |  |  | 21.00% |  |  |  | 9.00% |  |  |  |  |  |  |
Fair value | 2,549,000,000 |  | 2,549,000,000 |  | 6,954,000,000 | 413,000,000 |  |  |  |  |  |  |  |  |  |  | 723,000,000 | 323,000,000 | [1] |  | 450,000,000 | 18,000,000 | [1] |  | 774,000,000 | 135,000,000 | [1] |  |  |  |  |
Gross unrealized losses | (567,000,000) |  | (567,000,000) |  | (804,000,000) | 124,000,000 |  |  |  |  |  |  |  |  |  |  | (285,000,000) | 80,000,000 | [1] |  | 121,000,000 | 5,000,000 | [1] |  | 109,000,000 | 19,000,000 | [1] |  |  |  |  |
Other-than-temporary impairments | 11,000,000 | 0 | 44,000,000 | 43,000,000 |  |  | 1,000,000 | 0 | 30,000,000 | 39,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Benefit of Guarantees Provided |  |  |  |  |  |  |  |  |  |  | 21,000,000 | 259,000,000 | 119,000,000 | 54,000,000 |  |  |  |  |  | 323,000,000 |  |  |  | 18,000,000 |  |  |  | 114,000,000 |  |  |  |
Securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 239,000,000 |  |  |  | $ 257,000,000 |  |  |  | $ 132,000,000 |  |  |  |  |  |  |
Securities rated below investment grade percentage without giving effect to the benefit of guarantees provided by financial guarantors |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 33.00% |  |  |  | 57.00% |  |  |  | 17.00% |  |  |  |  |  |  |
[1] | Includes investments insured by MBIA Corp. and National.

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