Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 4) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Credit Derivatives
Net change in unrealized gains (losses)	$ 1,155,531,000	$ (276,427,000)	$ 829,836,000	$ 10,786,000
Credit Derivative Asset (Liability), net	(1,028,100,000)		(1,028,100,000)		(1,869,900,000)
Present Value of Expected Claim (Payments) Recoveries	(508,500,000)		(508,500,000)
Excess of the present value of future installment fees receivable	37,200,000		37,200,000
R & W included in credit derivative asset (liability)			213,000,000
Pooled corporate obligations
Credit Derivatives
Net change in unrealized gains (losses)	100,200,000	(16,000,000)	67,400,000	63,300,000
Credit Derivative Asset (Liability), net	(20,600,000)		(20,600,000)
Present Value of Expected Claim (Payments) Recoveries	(78,700,000)		(78,700,000)
CLOs/CBOs
Credit Derivatives
Net change in unrealized gains (losses)	13,100,000	(1,400,000)	11,500,000	1,900,000
Credit Derivative Asset (Liability), net	200,000		200,000
Synthetic investment grade pooled corporate
Credit Derivatives
Net change in unrealized gains (losses)	800,000	300,000	10,500,000	(3,700,000)
Credit Derivative Asset (Liability), net	(29,000,000)		(29,000,000)
Synthetic high yield pooled corporate
Credit Derivatives
Net change in unrealized gains (losses)	(1,300,000)	(2,900,000)	(600,000)	11,600,000
Credit Derivative Asset (Liability), net	(15,300,000)		(15,300,000)
Present Value of Expected Claim (Payments) Recoveries	(6,100,000)		(6,100,000)
Trust Preferred Securities Collateralized Debt Obligations Member
Credit Derivatives
Net change in unrealized gains (losses)	82,500,000	(11,600,000)	46,200,000	53,600,000
Credit Derivative Asset (Liability), net	20,300,000		20,300,000
Present Value of Expected Claim (Payments) Recoveries	(72,600,000)		(72,600,000)
Market value collateralized debt obligations of corporate obligations
Credit Derivatives
Net change in unrealized gains (losses)	5,100,000	(400,000)	(200,000)	(100,000)
Credit Derivative Asset (Liability), net	3,200,000		3,200,000
U.S. RMBS
Credit Derivatives
Net change in unrealized gains (losses)	999,600,000	(228,800,000)	657,700,000	(53,900,000)
Credit Derivative Asset (Liability), net	(402,000,000)		(402,000,000)
Present Value of Expected Claim (Payments) Recoveries	(327,400,000)		(327,400,000)
Option ARM and Alt-A first lien
Credit Derivatives
Net change in unrealized gains (losses)	780,800,000	(205,100,000)	541,300,000	(44,600,000)
Credit Derivative Asset (Liability), net	(354,900,000)		(354,900,000)
Present Value of Expected Claim (Payments) Recoveries	(219,300,000)		(219,300,000)
Subprime
Credit Derivatives
Net change in unrealized gains (losses)	108,600,000	(8,100,000)	17,300,000	(7,200,000)
Credit Derivative Asset (Liability), net	(29,600,000)		(29,600,000)
Present Value of Expected Claim (Payments) Recoveries	(113,800,000)		(113,800,000)
Prime first lien
Credit Derivatives
Net change in unrealized gains (losses)	101,900,000	(17,200,000)	89,700,000	2,200,000
Credit Derivative Asset (Liability), net	(1,500,000)		(1,500,000)
Closed end second lien and HELOCs
Credit Derivatives
Net change in unrealized gains (losses)	8,300,000	1,600,000	9,400,000	(4,300,000)
Credit Derivative Asset (Liability), net	(16,000,000)		(16,000,000)
Present Value of Expected Claim (Payments) Recoveries	5,700,000		5,700,000
CMBS
Credit Derivatives
Net change in unrealized gains (losses)	(200,000)	400,000	10,300,000	10,200,000
Credit Derivative Asset (Liability), net	(4,800,000)		(4,800,000)
Other:
Credit Derivatives
Net change in unrealized gains (losses)	55,900,000	(32,000,000)	94,400,000	(8,800,000)
Credit Derivative Asset (Liability), net	(600,700,000)		(600,700,000)
Present Value of Expected Claim (Payments) Recoveries	$ (102,400,000)		$ (102,400,000)