Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	$ 85,047,000,000	$ 109,771,000,000
Percentage of total	100.00%	100.00%
Unrealized Gain (Loss)	553,779,000	(155,106,000)	(337,810,000)
U.S. RMBS
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	8,532,000,000	9,776,000,000
Original Subordination (as a percent)	24.10%	23.10%
Current Subordination (as a percent)	32.20%	32.40%
Unrealized Gain (Loss)	381,300,000	(300,800,000)	(498,000,000)
U.S. RMBS | 2004 and Prior
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	144,000,000
Original Subordination (as a percent)	6.30%
Current Subordination (as a percent)	19.40%
U.S. RMBS | 2005
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	2,525,000,000
Original Subordination (as a percent)	30.50%
Current Subordination (as a percent)	64.80%
Unrealized Gain (Loss)	800,000
U.S. RMBS | 2006
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	1,641,000,000
Original Subordination (as a percent)	29.30%
Current Subordination (as a percent)	35.50%
Unrealized Gain (Loss)	5,200,000
U.S. RMBS | 2007
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	4,222,000,000
Original Subordination (as a percent)	18.60%
Current Subordination (as a percent)	11.00%
Unrealized Gain (Loss)	375,300,000
CMBS
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	4,612,000,000	6,751,000,000
Original Subordination (as a percent)	32.60%	29.80%
Current Subordination (as a percent)	38.90%	31.30%
Unrealized Gain (Loss)	10,400,000	10,100,000	(41,100,000)
CMBS | 2004 and Prior
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	173,000,000
Original Subordination (as a percent)	29.60%
Current Subordination (as a percent)	57.60%
Unrealized Gain (Loss)	(200,000)
CMBS | 2005
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	674,000,000
Original Subordination (as a percent)	17.90%
Current Subordination (as a percent)	32.60%
Unrealized Gain (Loss)	(100,000)
CMBS | 2006
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	2,176,000,000
Original Subordination (as a percent)	33.60%
Current Subordination (as a percent)	39.10%
Unrealized Gain (Loss)	12,100,000
CMBS | 2007
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	1,589,000,000
Original Subordination (as a percent)	38.00%
Current Subordination (as a percent)	39.30%
Unrealized Gain (Loss)	(1,400,000)
Super senior
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	21,802,000,000	29,344,000,000
Percentage of total	25.60%	26.70%
AAA
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	40,240,000,000	50,214,000,000
Percentage of total	47.30%	45.70%
AA
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	4,084,000,000	8,138,000,000
Percentage of total	4.80%	7.40%
A
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	5,830,000,000	7,405,000,000
Percentage of total	6.90%	6.70%
BBB
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	5,030,000,000	6,312,000,000
Percentage of total	5.90%	5.80%
Below investment grade ("BIG")
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	$ 8,061,000,000	$ 8,358,000,000
Percentage of total	9.50%	7.70%