Financial Guaranty Contracts Accounted for as Insurance (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Net Earned Premiums
Accretion of discount on net premiums receivable			$ 16,400,000	$ 23,700,000
Net earned premiums	230,068,000	292,110,000	484,045,000	611,670,000	1,186,700,000	930,400,000	261,400,000
Net earned premium related to consolidated FG VIEs	18,300,000	15,600,000	37,400,000	21,600,000
Gross premium receivable, net of ceding commissions payable:
Balance beginning of period	1,167,600,000	1,418,200,000	1,167,600,000	1,418,200,000
Change in accounting				(19,000,000)
Balance beginning of the period, adjusted			1,167,587,000	1,399,200,000	1,399,200,000
Premium written, net			102,900,000	178,700,000
Premium payments received, net			(151,700,000)	(234,300,000)
Adjustments to the premium receivable:
Changes in the expected term of financial guaranty insurance contracts			(91,100,000)	8,200,000
Accretion of discount			16,400,000	23,700,000
Foreign exchange translation			22,800,000	(65,900,000)
Other adjustments			(7,400,000)	1,700,000
Balance, end of period	1,059,461,000	1,311,300,000	1,059,461,000	1,311,300,000	1,167,587,000	1,399,200,000
Percentage of installment premium receivables denominated in currencies other than the U.S. dollar (as a percent)	51.00%		51.00%		42.00%
Gross premium collections expected:
Total gross expected collections related to VIE	31,700,000		31,700,000
Financial Guaranty
Net Earned Premiums
Scheduled net earned premiums	202,700,000	267,400,000	417,600,000	558,300,000
Acceleration of premium earnings	21,000,000	15,400,000	50,600,000	30,800,000
Accretion of discount on net premiums receivable	5,800,000	8,600,000	14,800,000	21,300,000
Total net earned premiums	229,500,000	291,400,000	483,000,000	610,400,000
Adjustments to the premium receivable:
Accretion of discount	5,800,000	8,600,000	14,800,000	21,300,000
Gross premium collections expected:
2011 (July 1 - September 30)	54,500,000		54,500,000
2011 (October 1 - December 31)	63,600,000		63,600,000
2012	117,500,000		117,500,000
2013	103,000,000		103,000,000
2014	91,100,000		91,100,000
2015	81,400,000		81,400,000
2016-2020	326,300,000		326,300,000
2021-2025	229,300,000		229,300,000
2026-2030	167,700,000		167,700,000
After 2030	213,900,000		213,900,000
Total gross expected collections	1,448,300,000		1,448,300,000
Other
Net Earned Premiums
Net earned premiums	$ 500,000	$ 700,000	$ 1,000,000	$ 1,300,000