Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 2) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	$ 83,002,000,000		$ 85,047,000,000
Percentage of total	100.00%		100.00%
Unrealized Gain (Loss)	(633,758,000)	(271,636,000)
U.S. RMBS
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	8,319,000,000		8,532,000,000
Original Subordination (as a percent)	23.50%		24.10%
Current Subordination (as a percent)	31.80%		32.20%
Unrealized Gain (Loss)	(629,400,000)	(290,800,000)
U.S. RMBS | 2004 and Prior
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	140,000,000
Original Subordination (as a percent)	6.40%
Current Subordination (as a percent)	19.40%
Unrealized Gain (Loss)	(1,500,000)
U.S. RMBS | 2005
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	2,444,000,000
Original Subordination (as a percent)	30.60%
Current Subordination (as a percent)	65.30%
Unrealized Gain (Loss)	(5,700,000)
U.S. RMBS | 2006
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	1,621,000,000
Original Subordination (as a percent)	29.40%
Current Subordination (as a percent)	35.50%
Unrealized Gain (Loss)	(33,400,000)
U.S. RMBS | 2007
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	4,114,000,000
Original Subordination (as a percent)	17.50%
Current Subordination (as a percent)	10.80%
Unrealized Gain (Loss)	(588,800,000)
CMBS
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	4,410,000,000		4,612,000,000
Original Subordination (as a percent)	33.50%		32.60%
Current Subordination (as a percent)	40.30%		38.90%
Unrealized Gain (Loss)	200,000	700,000
CMBS | 2004 and Prior
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	144,000,000
Original Subordination (as a percent)	28.20%
Current Subordination (as a percent)	59.20%
Unrealized Gain (Loss)	(100,000)
CMBS | 2005
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	672,000,000
Original Subordination (as a percent)	17.90%
Current Subordination (as a percent)	33.90%
Unrealized Gain (Loss)	(100,000)
CMBS | 2006
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	2,104,000,000
Original Subordination (as a percent)	33.90%
Current Subordination (as a percent)	39.80%
Unrealized Gain (Loss)	500,000
CMBS | 2007
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	1,490,000,000
Original Subordination (as a percent)	40.50%
Current Subordination (as a percent)	42.20%
Unrealized Gain (Loss)	(100,000)
Super senior
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	20,787,000,000		21,802,000,000
Percentage of total	25.00%		25.60%
AAA
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	40,099,000,000		40,240,000,000
Percentage of total	48.30%		47.30%
AA
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	3,617,000,000		4,084,000,000
Percentage of total	4.40%		4.80%
A
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	5,906,000,000		5,830,000,000
Percentage of total	7.10%		6.90%
BBB
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	5,114,000,000		5,030,000,000
Percentage of total	6.20%		5.90%
Below investment grade ("BIG")
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	$ 7,479,000,000		$ 8,061,000,000
Percentage of total	9.00%		9.50%