Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 5) (USD $)	3 Months Ended
	Mar. 31, 2012 Agreement Counterparty	Dec. 31, 2011
Components of Credit Derivative Assets (Liabilities)
Credit derivative assets	$ 463,556,000	$ 468,933,000
Credit derivative liabilities	(2,416,268,000)	(1,772,803,000)
Net fair value of credit derivatives	(1,952,700,000)	(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	(4,897,700,000)	(5,595,800,000)
Less: Effect of AGC and AGM credit spreads	(2,945,000,000)	(4,291,900,000)
Net fair value of credit derivatives	(1,952,700,000)	(1,303,900,000)
Other credit derivative disclosures
Gain from tightening of spreads	698,100,000
Par insured that could be terminated if ratings were downgraded	2,300,000,000
Number of ISDA master agreements	3
Par insured that could be terminated if ratings were downgraded to BBB- or Baa3	89,000,000
Number of counterparties that could terminate for ratings downgrade to BBB- or Baa3	1
Par insured that could be terminated if ratings were downgraded to BB+ or Ba1	2,200,000,000
Number of counterparties that could terminate for ratings downgrade to BB+ or Ba1	2
Amount of par subject to collateral posting	14,600,000,000
Collateral agreed to be posted	678,200,000
Amount of par subject to collateral for which the amount of collateral is capped	14,100,000,000
Cap on collateral requirement at current ratings levels	625,000,000
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+ or A3	$ 675,000,000
AGC
Credit Derivatives
Quoted price of CDS contract (as a percent)	7.43%	11.40%
AGM
Credit Derivatives
Quoted price of CDS contract (as a percent)	5.55%	7.78%