Financial Guaranty Insurance Contracts (Details 4) (USD $)							3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended			3 Months Ended																							12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2010	Mar. 31, 2012 Greek sovereign debt	Dec. 31, 2011 Greek sovereign debt	Mar. 31, 2012 U.S. dollar denominated obligations Minimum	Dec. 31, 2011 U.S. dollar denominated obligations Minimum	Mar. 31, 2012 U.S. dollar denominated obligations Maximum	Dec. 31, 2011 U.S. dollar denominated obligations Maximum	Mar. 31, 2012 Euro denominated obligations Minimum	Dec. 31, 2011 Euro denominated obligations Minimum	Mar. 31, 2012 Euro denominated obligations Maximum	Dec. 31, 2011 Euro denominated obligations Maximum	Mar. 31, 2012 Australian dollar denominated obligations Minimum	Dec. 31, 2011 Australian dollar denominated obligations Minimum	Mar. 31, 2012 Australian dollar denominated obligations Maximum	Dec. 31, 2011 Australian dollar denominated obligations Maximum	Mar. 31, 2012 Financial Guaranty	Mar. 31, 2011 Financial Guaranty	Dec. 31, 2011 Financial Guaranty	Mar. 31, 2012 Financial Guaranty U.S. RMBS	Mar. 31, 2011 Financial Guaranty U.S. RMBS	Mar. 31, 2012 Financial Guaranty Total first lien	Mar. 31, 2011 Financial Guaranty Total first lien	Mar. 31, 2012 Financial Guaranty Prime first lien	Mar. 31, 2011 Financial Guaranty Prime first lien	Mar. 31, 2012 Financial Guaranty Alt-A first lien	Mar. 31, 2011 Financial Guaranty Alt-A first lien	Mar. 31, 2012 Financial Guaranty Option ARM	Mar. 31, 2011 Financial Guaranty Option ARM	Mar. 31, 2012 Financial Guaranty Subprime	Mar. 31, 2011 Financial Guaranty Subprime	Mar. 31, 2012 Financial Guaranty Second lien	Mar. 31, 2011 Financial Guaranty Second lien	Mar. 31, 2012 Financial Guaranty CES	Mar. 31, 2011 Financial Guaranty CES	Mar. 31, 2012 Financial Guaranty HELOCs	Mar. 31, 2011 Financial Guaranty HELOCs	Mar. 31, 2012 Financial Guaranty Other structured finance	Mar. 31, 2011 Financial Guaranty Other structured finance	Mar. 31, 2012 Financial Guaranty Public finance	Mar. 31, 2011 Financial Guaranty Public finance	Mar. 31, 2012 Policies Paid in Installments	Dec. 31, 2011 Policies Paid in Installments	Mar. 31, 2012 Other	Dec. 31, 2011 Other
Selected Information for Policies Paid in Installments
Premiums receivable, net of ceding commissions payable	$ 1,018,672,000	$ 1,002,852,000	$ 1,118,000,000	$ 1,167,600,000																																								$ 1,018,700,000	$ 1,002,900,000
Gross deferred premium revenue																			5,918,800,000		6,046,300,000																							2,125,600,000	2,192,600,000
Weighted-average risk-free rate used to discount premiums (as a percent)																																												3.70%	3.40%
Weighted-average period of premiums receivable (in years)																																												10 years	9 years 9 months 18 days
Weighted-average risk free rate (as a percent)							0.00%	0.00%	3.94%	3.27%	0.00%	0.00%	2.84%	2.69%	0.00%	0.00%	4.82%	4.30%
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					231,900,000	42,600,000													638,000,000	408,800,000		319,200,000	160,800,000	429,900,000	909,900,000	1,800,000	1,400,000	134,900,000	184,400,000	152,900,000	523,700,000	140,300,000	200,400,000	(110,700,000)	(749,100,000)	(79,600,000)	56,600,000	(31,100,000)	(805,700,000)	252,800,000	159,100,000	66,000,000	88,900,000			1,900,000	1,900,000
Economic Loss Development																			204,800,000	(152,000,000)		7,900,000	(154,700,000)	1,400,000	(125,900,000)	400,000	100,000	(8,600,000)	6,500,000	(1,700,000)	(114,700,000)	11,300,000	(17,800,000)	6,500,000	(28,800,000)	(1,100,000)	(106,400,000)	7,600,000	77,600,000	(23,800,000)	16,300,000	220,700,000	(13,600,000)
(Paid) Recovered Losses																			(90,400,000)	(224,600,000)		(114,500,000)	(213,200,000)	(86,500,000)	(121,500,000)			(9,400,000)	(19,500,000)	(75,900,000)	(86,900,000)	(1,200,000)	(15,100,000)	(28,000,000)	(91,700,000)	(9,000,000)	(27,100,000)	(19,000,000)	(64,600,000)	(23,700,000)	(2,400,000)	47,800,000	(9,000,000)
Net Expected Loss to be Paid, at the end of the period					231,900,000	42,600,000													752,400,000	32,200,000		212,600,000	(207,100,000)	344,800,000	662,500,000	2,200,000	1,500,000	116,900,000	171,400,000	75,300,000	322,100,000	150,400,000	167,500,000	(132,200,000)	(869,600,000)	(89,700,000)	(76,900,000)	(42,500,000)	(792,700,000)	205,300,000	173,000,000	334,500,000	66,300,000			1,900,000	1,900,000
Period after the end of the reporting period within which the ceded paid losses are typically settled (in days)	45 days
Expected LAE for mitigating claim liabilities	$ 26,900,000	$ 35,500,000