Financial Guaranty Insurance Contracts (Details 4) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Selected Information for Policies Paid in Installments
Premiums receivable, net of ceding commissions payable	$ 1,002,852,000				$ 1,167,587,000				$ 1,002,852,000	$ 1,167,587,000	$ 1,418,200,000	$ 15,700,000
Rollforward of deferred acquisition costs
Balance, beginning of period				239,805,000				242,000,000	239,805,000	242,000,000	288,600,000
Change in accounting											101,800,000
Settlement of pre-existing relationships											(114,000,000)
Costs deferred during the period:
Ceded and assumed commissions									(12,500,000)	(18,200,000)	(10,200,000)
Premium taxes									6,600,000	11,600,000	14,200,000
Compensation and other acquisition costs									28,800,000	39,400,000	25,900,000
Total									22,900,000	32,800,000	29,900,000
Costs amortized during the period	(6,700,000)	(7,300,000)	(9,500,000)	(7,400,000)	(11,000,000)	(8,000,000)	(6,900,000)	(8,200,000)	(30,849,000)	(34,057,000)	(53,899,000)
Foreign exchange translation									0	(900,000)	(10,400,000)
Balance at the end of the period	231,875,000				239,805,000				231,875,000	239,805,000	242,000,000
Present Value of Net Expected Loss to be paid Roll Forward by Sector
Period after the end of the reporting period within which the ceded paid losses are typically settled (in days)	45 days								45 days
Expected LAE for mitigating claim liabilities	35,500,000				17,200,000				35,500,000	17,200,000
Minimum
Costs deferred during the period:
Weighted-average risk free rate (as a percent)									0.00%	0.00%
Maximum
Costs deferred during the period:
Weighted-average risk free rate (as a percent)									3.27%	5.34%
Financial Guaranty
Selected Information for Policies Paid in Installments
Gross deferred premium revenue	6,046,300,000				7,108,200,000				6,046,300,000	7,108,200,000
Present Value of Net Expected Loss to be paid Roll Forward by Sector
Net Expected Loss to be Paid, at the beginning of the period				408,800,000				1,066,400,000	408,800,000	1,066,400,000
Economic Loss Development									202,200,000	229,800,000
(Paid) Recovered Losses									27,000,000	(887,400,000)
Net Expected Loss to be Paid, at the end of the period	638,000,000				408,800,000				638,000,000	408,800,000
Financial Guaranty | U.S. RMBS
Present Value of Net Expected Loss to be paid Roll Forward by Sector
Net Expected Loss to be Paid, at the beginning of the period				160,800,000				819,800,000	160,800,000	819,800,000
Economic Loss Development									48,900,000	168,200,000
(Paid) Recovered Losses									109,500,000	(827,200,000)
Net Expected Loss to be Paid, at the end of the period	319,200,000				160,800,000				319,200,000	160,800,000
Financial Guaranty | Total first lien
Present Value of Net Expected Loss to be paid Roll Forward by Sector
Net Expected Loss to be Paid, at the beginning of the period				909,900,000				827,100,000	909,900,000	827,100,000
Economic Loss Development									(24,100,000)	327,800,000
(Paid) Recovered Losses									(455,900,000)	(245,000,000)
Net Expected Loss to be Paid, at the end of the period	429,900,000				909,900,000				429,900,000	909,900,000
Financial Guaranty | Prime first lien
Present Value of Net Expected Loss to be paid Roll Forward by Sector
Net Expected Loss to be Paid, at the beginning of the period				1,400,000					1,400,000
Economic Loss Development									400,000	1,400,000
Net Expected Loss to be Paid, at the end of the period	1,800,000				1,400,000				1,800,000	1,400,000
Financial Guaranty | Alt-A first lien
Present Value of Net Expected Loss to be paid Roll Forward by Sector
Net Expected Loss to be Paid, at the beginning of the period				184,400,000				204,400,000	184,400,000	204,400,000
Economic Loss Development									14,600,000	40,000,000
(Paid) Recovered Losses									(64,100,000)	(60,000,000)
Net Expected Loss to be Paid, at the end of the period	134,900,000				184,400,000				134,900,000	184,400,000
Financial Guaranty | Option ARM
Present Value of Net Expected Loss to be paid Roll Forward by Sector
Net Expected Loss to be Paid, at the beginning of the period				523,700,000				545,200,000	523,700,000	545,200,000
Economic Loss Development									3,900,000	160,100,000
(Paid) Recovered Losses									(374,700,000)	(181,600,000)
Net Expected Loss to be Paid, at the end of the period	152,900,000				523,700,000				152,900,000	523,700,000
Financial Guaranty | Subprime
Present Value of Net Expected Loss to be paid Roll Forward by Sector
Net Expected Loss to be Paid, at the beginning of the period				200,400,000				77,500,000	200,400,000	77,500,000
Economic Loss Development									(43,000,000)	126,300,000
(Paid) Recovered Losses									(17,100,000)	(3,400,000)
Net Expected Loss to be Paid, at the end of the period	140,300,000				200,400,000				140,300,000	200,400,000
Financial Guaranty | Second lien
Present Value of Net Expected Loss to be paid Roll Forward by Sector
Net Expected Loss to be Paid, at the beginning of the period				(749,100,000)				(7,300,000)	(749,100,000)	(7,300,000)
Economic Loss Development									73,000,000	(159,600,000)
(Paid) Recovered Losses									565,400,000	(582,200,000)
Net Expected Loss to be Paid, at the end of the period	(110,700,000)				(749,100,000)				(110,700,000)	(749,100,000)
Financial Guaranty | CES
Present Value of Net Expected Loss to be paid Roll Forward by Sector
Net Expected Loss to be Paid, at the beginning of the period				56,600,000				199,300,000	56,600,000	199,300,000
Economic Loss Development									(78,000,000)	(73,300,000)
(Paid) Recovered Losses									(58,200,000)	(69,400,000)
Net Expected Loss to be Paid, at the end of the period	(79,600,000)				56,600,000				(79,600,000)	56,600,000
Financial Guaranty | HELOCs
Present Value of Net Expected Loss to be paid Roll Forward by Sector
Net Expected Loss to be Paid, at the beginning of the period				(805,700,000)				(206,600,000)	(805,700,000)	(206,600,000)
Economic Loss Development									151,000,000	(86,300,000)
(Paid) Recovered Losses									623,600,000	(512,800,000)
Net Expected Loss to be Paid, at the end of the period	(31,100,000)				(805,700,000)				(31,100,000)	(805,700,000)
Financial Guaranty | Structured Finance
Present Value of Net Expected Loss to be paid Roll Forward by Sector
Net Expected Loss to be Paid, at the beginning of the period				159,100,000				115,700,000	159,100,000	115,700,000
Economic Loss Development									111,000,000	52,000,000
(Paid) Recovered Losses									(17,300,000)	(8,600,000)
Net Expected Loss to be Paid, at the end of the period	252,800,000				159,100,000				252,800,000	159,100,000
Financial Guaranty | Public finance
Present Value of Net Expected Loss to be paid Roll Forward by Sector
Net Expected Loss to be Paid, at the beginning of the period				88,900,000				130,900,000	88,900,000	130,900,000
Economic Loss Development									42,300,000	9,600,000
(Paid) Recovered Losses									(65,200,000)	(51,600,000)
Net Expected Loss to be Paid, at the end of the period	66,000,000				88,900,000				66,000,000	88,900,000
Policies Paid in Installments
Selected Information for Policies Paid in Installments
Premiums receivable, net of ceding commissions payable	1,002,900,000				1,167,600,000				1,002,900,000	1,167,600,000
Gross deferred premium revenue	2,192,600,000				2,933,600,000				2,192,600,000	2,933,600,000
Weighted-average risk-free rate used to discount premiums (as a percent)									3.40%	3.50%
Weighted-average period of premiums receivable (in years)									9.8	10.1
Other
Present Value of Net Expected Loss to be paid Roll Forward by Sector
Net Expected Loss to be Paid, at the end of the period	$ 1,900,000				$ 2,100,000				$ 1,900,000	$ 2,100,000