Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 2) (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
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	$ 98,005,000,000		$ 98,005,000,000		$ 109,771,000,000
Percentage of total (as a percent)	100.00%		100.00%		100.00%
Net change in unrealized gains (losses)	(48,625,000)	35,115,000	(319,751,000)	287,213,000	(157,800,000)	(337,800,000)	38,000,000
Super senior.
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	27,321,000,000		27,321,000,000		29,344,000,000
Percentage of total (as a percent)	27.90%		27.90%		26.70%
AAA.
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	44,074,000,000		44,074,000,000		50,214,000,000
Percentage of total (as a percent)	45.00%		45.00%		45.70%
AA.
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	6,392,000,000		6,392,000,000		8,138,000,000
Percentage of total (as a percent)	6.50%		6.50%		7.40%
A
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	6,506,000,000		6,506,000,000		7,405,000,000
Percentage of total (as a percent)	6.60%		6.60%		6.70%
BBB
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	5,276,000,000		5,276,000,000		6,312,000,000
Percentage of total (as a percent)	5.40%		5.40%		5.80%
Below investment grade ("BIG")
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	8,436,000,000		8,436,000,000		8,358,000,000
Percentage of total (as a percent)	8.60%		8.60%		7.70%
U.S. RMBS
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	9,147,000,000		9,147,000,000		9,776,000,000
Original Subordination (as a percent)	23.90%		23.90%		23.10%
Current Subordination (as a percent)	32.60%		32.60%		32.40%
Net change in unrealized gains (losses)	(45,700,000)	800,000	(336,000,000)	174,900,000
U.S. RMBS | 2004 and Prior
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	155,000,000		155,000,000
Original Subordination (as a percent)	6.20%		6.20%
Current Subordination (as a percent)	19.30%		19.30%
Net change in unrealized gains (losses)	300,000		(3,000,000)
U.S. RMBS | 2005
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	2,709,000,000		2,709,000,000
Original Subordination (as a percent)	30.30%		30.30%
Current Subordination (as a percent)	64.00%		64.00%
Net change in unrealized gains (losses)	1,900,000		(17,200,000)
U.S. RMBS | 2006
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	1,693,000,000		1,693,000,000
Original Subordination (as a percent)	29.10%		29.10%
Current Subordination (as a percent)	35.40%		35.40%
Net change in unrealized gains (losses)	(60,300,000)		(101,000,000)
U.S. RMBS | 2007
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	4,590,000,000		4,590,000,000
Original Subordination (as a percent)	18.60%		18.60%
Current Subordination (as a percent)	12.70%		12.70%
Net change in unrealized gains (losses)	12,400,000		(214,800,000)
CMBS
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	4,517,000,000		4,517,000,000		6,751,000,000
Original Subordination (as a percent)	33.80%		33.80%		29.80%
Current Subordination (as a percent)	39.40%		39.40%		31.30%
Net change in unrealized gains (losses)	9,800,000	300,000	10,500,000	9,800,000
CMBS | 2004 and Prior
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	241,000,000		241,000,000
Original Subordination (as a percent)	29.60%		29.60%
Current Subordination (as a percent)	56.80%		56.80%
Net change in unrealized gains (losses)	(100,000)		(200,000)
CMBS | 2005
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	677,000,000		677,000,000
Original Subordination (as a percent)	17.80%		17.80%
Current Subordination (as a percent)	26.40%		26.40%
CMBS | 2006
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	2,184,000,000		2,184,000,000
Original Subordination (as a percent)	33.50%		33.50%
Current Subordination (as a percent)	38.70%		38.70%
Net change in unrealized gains (losses)	10,300,000		10,900,000
CMBS | 2007
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	1,415,000,000		1,415,000,000
Original Subordination (as a percent)	42.60%		42.60%
Current Subordination (as a percent)	43.60%		43.60%
Net change in unrealized gains (losses)	$ (400,000)		$ (200,000)