Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 3) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2011	Jun. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Net Change in Fair Value of Credit Derivatives
Net credit derivative premiums received and receivable	$ 40,600,000		$ 49,800,000	$ 147,800,000	$ 154,200,000
Net ceding commissions (paid and payable) received and receivable	700,000		900,000	2,900,000	2,900,000
Realized gains on credit derivatives	41,300,000		50,700,000	150,700,000	157,100,000
Termination gains (losses)				(22,500,000)
Net credit derivative losses (paid and payable) recovered and recoverable	(40,800,000)		1,700,000	(103,100,000)	(39,600,000)
Total realized gains and other settlements on credit derivatives	483,000		52,498,000	25,074,000	117,554,000
Net unrealized gains and other settlements on credit derivatives	1,155,531,000		(276,427,000)	829,836,000	10,786,000
Net change in fair value of credit derivatives	1,156,014,000		(223,929,000)	854,910,000	128,340,000
Amount received on net par of terminated CDS exposures	2,600,000			24,200,000
Amount paid on net par of terminated CDS exposures		22,500,000
Net par of terminated CDS transactions	$ 1,800,000,000			$ 9,500,000,000