Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 5) (USD $)	6 Months Ended
	Jun. 30, 2012 contract	Dec. 31, 2011	Jun. 30, 2012 AGC	Mar. 31, 2012 AGC	Dec. 31, 2011 AGC	Jun. 30, 2012 AGM	Mar. 31, 2012 AGM	Dec. 31, 2011 AGM
Components of Credit Derivative Assets (Liabilities)
Quoted price of CDS contract (as a percent)			9.04%	7.43%	11.40%	6.52%	5.55%	7.78%
Credit Risk Derivative Assets, at Fair Value	$ 429,891,000	$ 468,933,000
Credit Risk Derivative Liabilities, at Fair Value	(2,095,852,000)	(1,772,803,000)
Net fair value of credit derivatives	(1,666,000,000)	(1,303,900,000)
Fair value of credit derivatives before effect of AGC and AGM credit spreads
Fair value of credit derivatives before effect of AGC and AGM credit spreads	(5,749,100,000)	(5,595,800,000)
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	4,083,100,000	4,291,900,000
Other credit derivative disclosures
Gain from tightening of spreads	153,300,000
Par insured that could be terminated if ratings were downgraded	2,200,000,000
Number of Isda Master Agreements That Counterparty Can Terminate Transaction Upon a Rating Downgrade	3
Par insured that could be terminated by one counterparty if ratings were downgraded to BBB- or Baa3	89,000,000
Par insured that could be terminated by two counterparties if ratings were downgraded to BB+ or Ba1	2,100,000,000
Amount of par subject to collateral posting	14,200,000,000
Collateral agreed to be posted	687,900,000
Amount of par subject to collateral for which the amount of collateral is capped	625,000,000
Cap on collateral requirement at current ratings levels	13,700,000,000
Increase to cap on collateral requirement if ratings are downgraded to A+ or A3	50,000,000
Cap on collateral requirement if ratings are downgraded to A plus or A3	$ 675,000,000