Financial Guaranty Contracts Accounted for as Insurance (Details 3) (USD $)	1 Months Ended		3 Months Ended		6 Months Ended			6 Months Ended																									6 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2009	Jun. 30, 2011 Public finance Financial Guaranty	Jun. 30, 2010 Public finance Financial Guaranty	Jun. 30, 2011 Total first lien Financial Guaranty	Jun. 30, 2010 Total first lien Financial Guaranty	Jun. 30, 2011 U.S. RMBS Financial Guaranty	Jun. 30, 2010 U.S. RMBS Financial Guaranty	Jun. 30, 2011 Prime first lien Financial Guaranty	Jun. 30, 2010 Prime first lien Financial Guaranty	Jun. 30, 2011 Alt-A first lien Financial Guaranty	Jun. 30, 2010 Alt-A first lien Financial Guaranty	Jun. 30, 2011 Option ARM Financial Guaranty	Jun. 30, 2010 Option ARM Financial Guaranty	Jun. 30, 2011 Subprime Financial Guaranty	Jun. 30, 2010 Subprime Financial Guaranty	Jun. 30, 2011 CES Financial Guaranty	Jun. 30, 2010 CES Financial Guaranty	Jun. 30, 2011 HELOCs Financial Guaranty	Jun. 30, 2010 HELOCs Financial Guaranty	Jun. 30, 2011 Second lien Financial Guaranty	Jun. 30, 2010 Second lien Financial Guaranty	Jun. 30, 2011 Other Structured finance. Financial Guaranty	Jun. 30, 2010 Other Structured finance. Financial Guaranty	Jun. 30, 2011 Financial Guaranty	Jun. 30, 2010 Financial Guaranty	Dec. 31, 2010 Financial Guaranty	Jun. 30, 2011 Policies Paid in Installments Y	Dec. 31, 2010 Policies Paid in Installments Y	Jun. 30, 2011 Other	Dec. 31, 2010 Other
Selected Information for Policies Paid in Installments
Premiums receivable, net of ceding commissions payable	$ 1,059,461,000	$ 1,167,587,000	$ 1,059,461,000	$ 1,311,300,000	$ 1,059,461,000	$ 1,311,300,000	$ 1,399,200,000																										$ 1,059,500,000	$ 1,167,600,000
Gross deferred premium revenue																														6,412,600,000		7,108,600,000	2,384,400,000	2,933,600,000
Weighted-average risk-free rate used to discount premiums (as a percent)																																	3.60%	3.50%
Weighted-average period of premiums receivable (in years)																																	10.1	10.1
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	909,900,000	827,100,000	160,800,000	793,500,000	1,400,000		184,400,000	204,400,000	523,700,000	545,200,000	200,400,000	77,500,000	56,600,000	199,300,000	(805,700,000)	(232,900,000)	(749,100,000)	(33,600,000)	145,100,000	102,600,000	394,800,000	1,027,000,000				2,100,000	2,100,000
Development Economic Loss								(13,500,000)	(8,100,000)	(88,000,000)	160,200,000	(92,900,000)	174,800,000	1,800,000	400,000	21,800,000	15,400,000	(88,400,000)	75,100,000	(23,200,000)	69,300,000	(109,600,000)	(40,400,000)	104,700,000	55,000,000	(4,900,000)	14,600,000	38,500,000	35,500,000	(67,900,000)	202,200,000
Less: (Paid) Recovered Losses			564,700,000	(206,000,000)	398,300,000	(417,000,000)		(9,200,000)	(34,200,000)	(222,700,000)	(80,400,000)	398,300,000	(436,100,000)			(38,600,000)	(29,000,000)	(168,400,000)	(49,100,000)	(15,700,000)	(2,300,000)	(41,700,000)	(39,900,000)	662,700,000	(315,800,000)	621,000,000	(355,700,000)	(3,000,000)	(5,600,000)	386,100,000	(475,900,000)
Net Expected Loss to be Paid, at the end of the period								66,200,000	88,600,000	599,200,000	906,900,000	466,200,000	532,200,000	3,200,000	400,000	167,600,000	190,800,000	266,900,000	571,200,000	161,500,000	144,500,000	(94,700,000)	119,000,000	(38,300,000)	(493,700,000)	(133,000,000)	(374,700,000)	180,600,000	132,500,000	713,000,000	753,300,000				2,100,000	2,100,000
Expected LAE for mitigating claim liabilities	$ 15,900,000	$ 17,200,000	$ 15,900,000		$ 15,900,000
Weighted-average risk free rate to discount expected loss to be paid, low end of range (as a percent)	0.00%	0.00%
Weighted-average risk free rate to discount expected loss to be paid, high end of range (as a percent)	5.00%	5.34%