Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 2) (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011	Jun. 30, 2012		Jun. 30, 2011	Dec. 31, 2011
Distribution of Credit Derivative Net Par Outstanding
Premium written, net				$ 102,700,000		$ 102,900,000
Net Par Outstanding	78,443,000,000			78,443,000,000			85,047,000,000
Percentage of total	100.00%			100.00%			100.00%
Original Subordination (as a percent)	0.00%	[1]		0.00%	[1]
Current Subordination (as a percent)	0.00%	[1]		0.00%	[1]		0.00%	[1]
Unrealized Gains (Losses)	283,327,000		(54,059,000)	(350,431,000)		(325,695,000)
U.S. RMBS
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	7,902,000,000			7,902,000,000			8,532,000,000
Original Subordination (as a percent)	24.10%	[1],[2]		24.10%	[1],[2]		24.10%	[1]
Current Subordination (as a percent)	31.50%	[1],[2]		31.50%	[1],[2]		32.20%	[1]
Unrealized Gains (Losses)	249,700,000		(51,100,000)	(379,700,000)		(341,900,000)
U.S. RMBS | 2004 and Prior
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	136,000,000			136,000,000
Original Subordination (as a percent)	6.40%	[2]		6.40%	[2]
Current Subordination (as a percent)	19.40%	[2]		19.40%	[2]
Unrealized Gains (Losses)	1,600,000			100,000
U.S. RMBS | 2005
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	2,316,000,000			2,316,000,000
Original Subordination (as a percent)	30.80%	[2]		30.80%	[2]
Current Subordination (as a percent)	65.80%	[2]		65.80%	[2]
Unrealized Gains (Losses)	5,100,000			(600,000)
U.S. RMBS | Vintage 2006 [Member]
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	1,602,000,000			1,602,000,000
Original Subordination (as a percent)	29.40%	[2]		29.40%	[2]
Current Subordination (as a percent)	35.10%	[2]		35.10%	[2]
Unrealized Gains (Losses)	18,900,000			(14,500,000)
U.S. RMBS | Vintage 2007 [Member]
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	3,848,000,000			3,848,000,000
Original Subordination (as a percent)	18.40%	[2]		18.40%	[2]
Current Subordination (as a percent)	9.80%	[2]		9.80%	[2]
Unrealized Gains (Losses)	224,100,000			(364,700,000)
CMBS
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	4,270,000,000			4,270,000,000			4,612,000,000
Original Subordination (as a percent)	33.50%	[1],[2]		33.50%	[1],[2]		32.60%	[1]
Current Subordination (as a percent)	41.00%	[1],[2]		41.00%	[1],[2]		38.90%	[1]
Unrealized Gains (Losses)	100,000		9,800,000	300,000		10,500,000
CMBS | 2004 and Prior
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	80,000,000			80,000,000
Original Subordination (as a percent)	27.40%	[2]		27.40%	[2]
Current Subordination (as a percent)	66.60%	[2]		66.60%	[2]
Unrealized Gains (Losses)	0			(100,000)
CMBS | 2005
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	669,000,000			669,000,000
Original Subordination (as a percent)	18.00%	[2]		18.00%	[2]
Current Subordination (as a percent)	35.00%	[2]		35.00%	[2]
Unrealized Gains (Losses)	(100,000)			(200,000)
CMBS | Vintage 2006 [Member]
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	2,077,000,000			2,077,000,000
Original Subordination (as a percent)	34.00%	[2]		34.00%	[2]
Current Subordination (as a percent)	40.70%	[2]		40.70%	[2]
Unrealized Gains (Losses)	400,000			900,000
CMBS | Vintage 2007 [Member]
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	1,444,000,000			1,444,000,000
Original Subordination (as a percent)	40.20%	[2]		40.20%	[2]
Current Subordination (as a percent)	42.70%	[2]		42.70%	[2]
Unrealized Gains (Losses)	(200,000)			(300,000)
Super senior
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	19,676,000,000			19,676,000,000			21,802,000,000
Percentage of total	25.10%			25.10%			25.60%
AAA
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	37,504,000,000			37,504,000,000			40,240,000,000
Percentage of total	47.80%			47.80%			47.30%
AA
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	3,500,000,000			3,500,000,000			4,342,000,000
Percentage of total	4.50%			4.50%			5.10%
A
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	5,887,000,000			5,887,000,000			5,830,000,000
Percentage of total	7.50%			7.50%			6.90%
BBB
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	4,776,000,000			4,776,000,000			5,030,000,000
Percentage of total	6.10%			6.10%			5.90%
Below investment grade ("BIG")
Distribution of Credit Derivative Net Par Outstanding
Net Par Outstanding	$ 7,100,000,000			$ 7,100,000,000			$ 7,803,000,000
Percentage of total	9.00%			9.00%			9.20%

[1] esents the sum of subordinate tranches and over-collateralization and does not include any benefit from excess interest collections that may be used to absorb losses. E

[2] esents the sum of subordinate tranches and overcollateralization and does not include any benefit from excess interest collections that may be used to absorb losses.Cre