Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 4) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Credit Derivatives
Net change in unrealized gains (losses)	$ (54,059,000)	$ 35,115,000	$ (325,695,000)	$ 287,213,000
Credit Derivative Asset (Liability), net	(2,187,500,000)		(2,187,500,000)		(1,869,900,000)
Present Value of Expected Claim (Payments) Recoveries	(537,200,000)		(537,200,000)
Excess of the present value of future installment fees receivable	63,800,000		63,800,000
Pooled corporate obligations
Credit Derivatives
Net change in unrealized gains (losses)	(21,600,000)	34,900,000	(32,800,000)	79,300,000
Credit Derivative Asset (Liability), net	(120,100,000)		(120,100,000)
Present Value of Expected Claim (Payments) Recoveries	(62,200,000)		(62,200,000)
CLOs/CBOs
Credit Derivatives
Net change in unrealized gains (losses)	(3,600,000)	1,800,000	(1,600,000)	3,300,000
Credit Derivative Asset (Liability), net	(12,000,000)		(12,000,000)
Synthetic investment grade pooled corporate
Credit Derivatives
Net change in unrealized gains (losses)	(800,000)	3,600,000	9,700,000	(4,000,000)
Credit Derivative Asset (Liability), net	(29,800,000)		(29,800,000)
Synthetic high yield pooled corporate
Credit Derivatives
Net change in unrealized gains (losses)	3,500,000	(5,900,000)	700,000	14,500,000
Credit Derivative Asset (Liability), net	(14,300,000)		(14,300,000)
Present Value of Expected Claim (Payments) Recoveries	(5,500,000)		(5,500,000)
Trust preferred securities collaralized debt obligations
Credit Derivatives
Net change in unrealized gains (losses)	(15,500,000)	35,500,000	(36,300,000)	65,200,000
Credit Derivative Asset (Liability), net	(62,100,000)		(62,100,000)
Present Value of Expected Claim (Payments) Recoveries	(56,700,000)		(56,700,000)
Market value collateralized debt obligations of corporate obligations
Credit Derivatives
Net change in unrealized gains (losses)	(5,200,000)	(100,000)	(5,300,000)	300,000
Credit Derivative Asset (Liability), net	(1,900,000)		(1,900,000)
U.S. RMBS
Credit Derivatives
Net change in unrealized gains (losses)	(51,100,000)	800,000	(341,900,000)	174,900,000
Credit Derivative Asset (Liability), net	(1,401,400,000)		(1,401,400,000)
Present Value of Expected Claim (Payments) Recoveries	(383,400,000)		(383,400,000)
Option ARM and Alt-A first lien
Credit Derivatives
Net change in unrealized gains (losses)	28,100,000	9,600,000	(239,500,000)	160,500,000
Credit Derivative Asset (Liability), net	(1,162,100,000)		(1,162,100,000)
Present Value of Expected Claim (Payments) Recoveries	(263,500,000)		(263,500,000)
Subprime
Credit Derivatives
Net change in unrealized gains (losses)	(67,200,000)	300,000	(91,300,000)	900,000
Credit Derivative Asset (Liability), net	(111,600,000)		(111,600,000)
Present Value of Expected Claim (Payments) Recoveries	(124,500,000)		(124,500,000)
Prime first lien
Credit Derivatives
Net change in unrealized gains (losses)	(12,800,000)	5,200,000	(12,200,000)	19,400,000
Credit Derivative Asset (Liability), net	(103,400,000)		(103,400,000)
Closed end second lien and HELOCs
Credit Derivatives
Net change in unrealized gains (losses)	800,000	(14,300,000)	1,100,000	(5,900,000)
Credit Derivative Asset (Liability), net	(24,300,000)		(24,300,000)
Present Value of Expected Claim (Payments) Recoveries	4,600,000		4,600,000
CMBS
Credit Derivatives
Net change in unrealized gains (losses)	9,800,000	300,000	10,500,000	9,800,000
Credit Derivative Asset (Liability), net	(4,500,000)		(4,500,000)
Other:
Credit Derivatives
Net change in unrealized gains (losses)	8,900,000	(900,000)	38,600,000	23,200,000
Credit Derivative Asset (Liability), net	(661,500,000)		(661,500,000)
Present Value of Expected Claim (Payments) Recoveries	$ (91,600,000)		$ (91,600,000)