Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 3) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Net Change in Fair Value of Credit Derivatives
Net credit derivative premiums received and receivable	$ 47,600,000	$ 50,700,000	$ 107,200,000	$ 104,400,000
Net ceding commissions (paid and payable) received and receivable	800,000	1,100,000	2,200,000	2,100,000
Realized gains on credit derivatives	48,400,000	51,800,000	109,400,000	106,500,000
Termination gains (losses)	(22,500,000)		(22,500,000)
Net credit derivative losses (paid and payable) recovered and recoverable	(36,700,000)	(13,400,000)	(62,300,000)	(41,400,000)
Total realized gains and other settlements on credit derivatives	(10,836,000)	38,353,000	24,591,000	65,056,000	153,500,000	163,600,000	117,600,000
Net unrealized gains and other settlements on credit derivatives	(48,625,000)	35,115,000	(319,751,000)	287,213,000	(157,800,000)	(337,800,000)	38,000,000
Net change in fair value of credit derivatives	(59,461,000)	73,468,000	(295,160,000)	352,269,000	(4,300,000)	(174,200,000)	155,600,000
Amount received on net par of terminated CDS exposures	6,100,000		21,600,000
Amount paid on net par of terminated CDS exposures			22,500,000
Claim payments on credit derivatives	$ 30,200,000	$ 13,000,000	$ 43,200,000	$ 41,000,000