Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 3) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Net credit derivative premiums received and receivable	$ 34,200,000	$ 47,600,000	$ 63,100,000	$ 107,200,000
Net ceding commissions (paid and payable) received and receivable	(100,000)	800,000	(200,000)	2,200,000
Realized gains on credit derivatives	34,100,000	48,400,000	62,900,000	109,400,000
Termination losses	(500,000)	(22,500,000)	(500,000)	(22,500,000)
Net credit derivative losses (paid and payable) recovered and recoverable	(56,300,000)	(36,700,000)	(142,000,000)	(62,300,000)
Unrealized Gains (Losses)	283,327,000	(54,059,000)	(350,431,000)	(325,695,000)
Realized Gains and Other Settlements	(22,664,000)	(10,836,000)	(79,545,000)	24,591,000
Net change in fair value of credit derivatives	260,663,000	(64,895,000)	(429,976,000)	(301,104,000)
Net par of terminated CDS transactions	600,000,000		800,000,000
Amount received on net par of terminated CDS exposures		$ 5,200,000,000		$ 7,700,000,000