Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 4) (USD $)	3 Months Ended			6 Months Ended			12 Months Ended
	Jun. 30, 2012		Jun. 30, 2011	Jun. 30, 2012		Jun. 30, 2011	Dec. 31, 2011
Credit Derivatives
Unrealized Gains (Losses)	$ 283,327,000		$ (54,059,000)	$ (350,431,000)		$ (325,695,000)
Net fair value of credit derivatives	(1,666,000,000)			(1,666,000,000)			(1,303,900,000)
Present Value of Expected Claim Payments Recoveries	(321,700,000)	[1]		(321,700,000)	[1]		(418,900,000)	[1]
Present Value of Future Installment Fees Receivable	53,600,000			53,600,000			47,100,000
R and W Included in Credit Derivative Asset (Liability)				240,700,000			215,000,000
AGC R&W Benefits From Deutsche Bank Settlement [Member]
Credit Derivatives
Unrealized Gain (Loss) on Derivatives	84,700,000
Pooled corporate obligations
Credit Derivatives
Unrealized Gains (Losses)	24,400,000		(21,600,000)	29,900,000		(32,800,000)
Net fair value of credit derivatives	(23,200,000)			(23,200,000)			(49,600,000)
Present Value of Expected Claim Payments Recoveries	(33,200,000)	[1]		(33,200,000)	[1]		(44,500,000)	[1]
CLOs/CBOs
Credit Derivatives
Unrealized Gains (Losses)	0		(3,600,000)	7,300,000		(1,600,000)
Net fair value of credit derivatives	5,600,000			5,600,000			(700,000)
Present Value of Expected Claim Payments Recoveries	0	[1]		0	[1]		0	[1]
Synthetic investment grade pooled corporate
Credit Derivatives
Unrealized Gains (Losses)	7,200,000		(800,000)	8,800,000		9,700,000
Net fair value of credit derivatives	(15,300,000)			(15,300,000)			(23,800,000)
Present Value of Expected Claim Payments Recoveries	0	[1]		0	[1]		0	[1]
Synthetic high yield pooled corporate
Credit Derivatives
Unrealized Gains (Losses)	(200,000)		3,500,000	10,600,000		700,000
Net fair value of credit derivatives	(5,800,000)			(5,800,000)			(15,700,000)
Present Value of Expected Claim Payments Recoveries	0	[1]		0	[1]		(5,200,000)	[1]
Trust Preferred Securities Collateralized Debt Obligations [Member]
Credit Derivatives
Unrealized Gains (Losses)	17,300,000		(15,500,000)	3,500,000		(36,300,000)
Net fair value of credit derivatives	(8,600,000)			(8,600,000)			(11,900,000)
Present Value of Expected Claim Payments Recoveries	(33,200,000)	[1]		(33,200,000)	[1]		(39,300,000)	[1]
Market value collateralized debt obligations of corporate obligations
Credit Derivatives
Unrealized Gains (Losses)	100,000		(5,200,000)	(300,000)		(5,300,000)
Net fair value of credit derivatives	900,000			900,000			2,500,000
Present Value of Expected Claim Payments Recoveries	0	[1]		0	[1]		0	[1]
U.S. RMBS
Credit Derivatives
Unrealized Gains (Losses)	249,700,000		(51,100,000)	(379,700,000)		(341,900,000)
Net fair value of credit derivatives	(1,065,400,000)			(1,065,400,000)			(678,100,000)
Present Value of Expected Claim Payments Recoveries	(191,300,000)	[1]		(191,300,000)	[1]		(279,500,000)	[1]
Option ARM and Alt-A first lien
Credit Derivatives
Unrealized Gains (Losses)	174,700,000		28,100,000	(343,000,000)		(239,500,000)
Net fair value of credit derivatives	(953,000,000)	[2]		(953,000,000)	[2]		(596,400,000)	[2]
Present Value of Expected Claim Payments Recoveries	(132,900,000)	[1],[2]		(132,900,000)	[1],[2]		(191,200,000)	[1],[2]
Subprime
Credit Derivatives
Unrealized Gains (Losses)	39,000,000		(67,200,000)	12,900,000		(91,300,000)
Net fair value of credit derivatives	(3,700,000)			(3,700,000)			(22,500,000)
Present Value of Expected Claim Payments Recoveries	(70,100,000)	[1]		(70,100,000)	[1]		(94,900,000)	[1]
Prime first lien
Credit Derivatives
Unrealized Gains (Losses)	34,800,000		(12,800,000)	(51,300,000)		(12,200,000)
Net fair value of credit derivatives	(95,700,000)			(95,700,000)			(44,300,000)
Present Value of Expected Claim Payments Recoveries	0	[1]		0	[1]		0	[1]
Closed end second lien and HELOCs
Credit Derivatives
Unrealized Gains (Losses)	1,200,000		800,000	1,700,000		1,100,000
Net fair value of credit derivatives	(13,000,000)			(13,000,000)			(14,900,000)
Present Value of Expected Claim Payments Recoveries	11,700,000	[1]		11,700,000	[1]		6,600,000	[1]
CMBS
Credit Derivatives
Unrealized Gains (Losses)	100,000		9,800,000	300,000		10,500,000
Net fair value of credit derivatives	(4,300,000)			(4,300,000)			(4,900,000)
Present Value of Expected Claim Payments Recoveries	0	[1]		0	[1]		0	[1]
Other
Credit Derivatives
Unrealized Gains (Losses)	9,200,000		8,900,000	(900,000)		38,600,000
Net fair value of credit derivatives	(573,100,000)			(573,100,000)			(571,300,000)
Present Value of Expected Claim Payments Recoveries	$ (97,200,000)	[1]		$ (97,200,000)	[1]		$ (94,900,000)	[1]

[1] esents amount in excess of the present value of future installment fees to be received of $53.6 million as of June 30, 2012 and $47.1 million as of December 31, 2011. Includes R&W benefit of $240.7 million as of June 30, 2012 and $215.0 million as of December 31, 2011. (2) Includes

[2] udes one transaction which is covered under the Bank of America Agreement. R