Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 3) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Net Change in Fair Value of Credit Derivatives
Net credit derivative premiums received and receivable									$ 184,700,000	$ 206,800,000	$ 168,100,000
Net ceding commissions (paid and payable) received and receivable									3,400,000	3,500,000	2,200,000
Realized gains on credit derivatives									188,100,000	210,300,000	170,300,000
Termination gains (losses)									(22,500,000)
Net credit derivative losses (paid and payable) recovered and recoverable									(159,600,000)	(56,800,000)	(6,700,000)
Total realized gains and other settlements on credit derivatives									5,959,000	153,495,000	163,558,000
Net unrealized gains and other settlements on credit derivatives									553,779,000	(155,106,000)	(337,810,000)
Net change in fair value of credit derivatives	(295,200,000)	1,155,900,000	(64,800,000)	(236,200,000)	(129,900,000)	(224,000,000)	73,500,000	278,800,000	559,738,000	(1,611,000)	(174,252,000)
Amount received on net par of terminated CDS exposures									24,700,000
Amount paid on net par of terminated CDS exposures									22,500,000
Net par of terminated CDS transactions									$ 11,500,000,000