Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 4) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Credit Derivatives
Net change in unrealized gains (losses)	$ (633,758,000)	$ (271,636,000)
Credit Derivative Asset (Liability), net	(1,952,700,000)		(1,303,900,000)
Present Value of Expected Claim (Payments) Recoveries	(376,200,000)		(418,900,000)
Present value of future installment fees receivable	54,400,000		47,100,000
R & W included in credit derivative asset (liability)	233,400,000		215,000,000
Pooled corporate obligations
Credit Derivatives
Net change in unrealized gains (losses)	5,500,000	(11,200,000)
Credit Derivative Asset (Liability), net	(44,400,000)		(49,600,000)
Present Value of Expected Claim (Payments) Recoveries	(38,700,000)		(44,500,000)
CLOs/CBOs
Credit Derivatives
Net change in unrealized gains (losses)	7,300,000	2,000,000
Credit Derivative Asset (Liability), net	6,600,000		(700,000)
Synthetic investment grade pooled corporate
Credit Derivatives
Net change in unrealized gains (losses)	1,600,000	10,500,000
Credit Derivative Asset (Liability), net	(22,600,000)		(23,800,000)
Synthetic high yield pooled corporate
Credit Derivatives
Net change in unrealized gains (losses)	10,800,000	(2,800,000)
Credit Derivative Asset (Liability), net	(4,900,000)		(15,700,000)
Present Value of Expected Claim (Payments) Recoveries	(600,000)		(5,200,000)
Trust preferred securities collateralized debt obligations
Credit Derivatives
Net change in unrealized gains (losses)	(13,800,000)	(20,800,000)
Credit Derivative Asset (Liability), net	(25,800,000)		(11,900,000)
Present Value of Expected Claim (Payments) Recoveries	(38,100,000)		(39,300,000)
Market value collateralized debt obligations of corporate obligations
Credit Derivatives
Net change in unrealized gains (losses)	(400,000)	(100,000)
Credit Derivative Asset (Liability), net	2,300,000		2,500,000
U.S. RMBS
Credit Derivatives
Net change in unrealized gains (losses)	(629,400,000)	(290,800,000)
Credit Derivative Asset (Liability), net	(1,313,600,000)		(678,100,000)
Present Value of Expected Claim (Payments) Recoveries	(241,200,000)		(279,500,000)
Option ARM and Alt-A first lien
Credit Derivatives
Net change in unrealized gains (losses)	(517,700,000)	(267,600,000)
Credit Derivative Asset (Liability), net	(1,137,200,000)		(596,400,000)
Present Value of Expected Claim (Payments) Recoveries	(159,400,000)		(191,200,000)
Subprime
Credit Derivatives
Net change in unrealized gains (losses)	(26,100,000)	(24,100,000)
Credit Derivative Asset (Liability), net	(31,600,000)		(22,500,000)
Present Value of Expected Claim (Payments) Recoveries	(94,600,000)		(94,900,000)
Prime first lien
Credit Derivatives
Net change in unrealized gains (losses)	(86,100,000)	600,000
Credit Derivative Asset (Liability), net	(130,400,000)		(44,300,000)
Closed end second lien and HELOCs
Credit Derivatives
Net change in unrealized gains (losses)	500,000	300,000
Credit Derivative Asset (Liability), net	(14,400,000)		(14,900,000)
Present Value of Expected Claim (Payments) Recoveries	12,800,000		6,600,000
CMBS
Credit Derivatives
Net change in unrealized gains (losses)	200,000	700,000
Credit Derivative Asset (Liability), net	(4,600,000)		(4,900,000)
Other:
Credit Derivatives
Net change in unrealized gains (losses)	(10,100,000)	29,700,000
Credit Derivative Asset (Liability), net	(590,100,000)		(571,300,000)
Present Value of Expected Claim (Payments) Recoveries	$ (96,300,000)		$ (94,900,000)