Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 4) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Credit Derivatives
Net change in unrealized gains (losses)	$ 553,779,000	$ (155,106,000)	$ (337,810,000)
Credit Derivative Asset (Liability), net	(1,303,900,000)	(1,869,900,000)
Present Value of Expected Claim (Payments) Recoveries	(418,900,000)
Present value of future installment fees receivable	47,100,000
R & W included in credit derivative asset (liability)	215,000,000
Pooled corporate obligations
Credit Derivatives
Net change in unrealized gains (losses)	38,800,000	70,000,000	194,600,000
Credit Derivative Asset (Liability), net	(49,600,000)
Present Value of Expected Claim (Payments) Recoveries	(44,500,000)
CLOs/CBOs
Credit Derivatives
Net change in unrealized gains (losses)	10,400,000	2,100,000	152,300,000
Credit Derivative Asset (Liability), net	(700,000)
Synthetic investment grade pooled corporate
Credit Derivatives
Net change in unrealized gains (losses)	15,600,000	(1,900,000)	(24,000,000)
Credit Derivative Asset (Liability), net	(23,800,000)
Synthetic high yield pooled corporate
Credit Derivatives
Net change in unrealized gains (losses)	(1,200,000)	10,800,000	104,700,000
Credit Derivative Asset (Liability), net	(15,700,000)
Present Value of Expected Claim (Payments) Recoveries	(5,200,000)
Trust preferred securities collateralized debt obligations
Credit Derivatives
Net change in unrealized gains (losses)	14,300,000	59,100,000	(44,100,000)
Credit Derivative Asset (Liability), net	(11,900,000)
Present Value of Expected Claim (Payments) Recoveries	(39,300,000)
Market value collateralized debt obligations of corporate obligations
Credit Derivatives
Net change in unrealized gains (losses)	(300,000)	(100,000)	(600,000)
Credit Derivative Asset (Liability), net	2,500,000
CDO of CDOs (corporate)
Credit Derivatives
Net change in unrealized gains (losses)			6,300,000
U.S. RMBS
Credit Derivatives
Net change in unrealized gains (losses)	381,300,000	(300,800,000)	(498,000,000)
Credit Derivative Asset (Liability), net	(678,100,000)
Present Value of Expected Claim (Payments) Recoveries	(279,500,000)
Option ARM and Alt-A first lien
Credit Derivatives
Net change in unrealized gains (losses)	299,900,000	(280,400,000)	(429,300,000)
Credit Derivative Asset (Liability), net	(596,400,000)
Present Value of Expected Claim (Payments) Recoveries	(191,200,000)
Subprime
Credit Derivatives
Net change in unrealized gains (losses)	24,000,000	(10,100,000)	4,900,000
Credit Derivative Asset (Liability), net	(22,500,000)
Present Value of Expected Claim (Payments) Recoveries	(94,900,000)
Prime first lien
Credit Derivatives
Net change in unrealized gains (losses)	46,900,000	(8,300,000)	(85,200,000)
Credit Derivative Asset (Liability), net	(44,300,000)
Closed end second lien and HELOCs
Credit Derivatives
Net change in unrealized gains (losses)	10,500,000	(2,000,000)	11,600,000
Credit Derivative Asset (Liability), net	(14,900,000)
Present Value of Expected Claim (Payments) Recoveries	6,600,000
CMBS
Credit Derivatives
Net change in unrealized gains (losses)	10,400,000	10,100,000	(41,100,000)
Credit Derivative Asset (Liability), net	(4,900,000)
Other:
Credit Derivatives
Net change in unrealized gains (losses)	123,200,000	65,600,000	6,700,000
Credit Derivative Asset (Liability), net	(571,300,000)
Present Value of Expected Claim (Payments) Recoveries	$ (94,900,000)