Financial Guaranty Insurance Contracts (Details 9) (USD $)	12 Months Ended				12 Months Ended					12 Months Ended								12 Months Ended					12 Months Ended
	Dec. 31, 2011 Agreement Transaction	Dec. 31, 2010 Transaction	Dec. 31, 2011 Bank of America agreement	Apr. 14, 2011 Bank of America agreement Agreement	Dec. 31, 2011 U.S. RMBS policy	Dec. 31, 2010 U.S. RMBS policy	Dec. 31, 2011 Alt-A first lien policy	Dec. 31, 2010 Alt-A first lien policy	Dec. 31, 2011 Total first lien loan	Dec. 31, 2011 Total first lien Bank of America agreement Transaction	Dec. 31, 2011 Prime first lien policy	Dec. 31, 2010 Prime first lien policy	Dec. 31, 2011 Option ARM policy	Dec. 31, 2010 Option ARM policy	Dec. 31, 2011 Subprime policy	Dec. 31, 2010 Subprime policy	Dec. 31, 2011 Second lien loan	Dec. 31, 2011 Second lien Bank of America agreement Transaction	Dec. 31, 2011 Closed end second lien and HELOCs Minimum	Dec. 31, 2010 Closed end second lien and HELOCs Minimum	Dec. 31, 2011 Closed end second lien and HELOCs Maximum	Dec. 31, 2010 Closed end second lien and HELOCs Maximum	Dec. 31, 2011 CES policy	Dec. 31, 2010 CES policy	Dec. 31, 2011 HELOCs policy	Dec. 31, 2010 HELOCs policy
Financial guarantee obligations
Number of negotiated agreements	2			1
Number of defaulted loan files reviewed									18,500								16,000
Outstanding par of defaulted loans reviewed									$ 5,200,000,000								$ 1,100,000,000
Number of loan files that breached one or more R&W									16,700								15,000
Threshold days delinquent for which all loan files are requested for review (in days)	60 days
Amount of loans to be repurchased by providers									70,200,000								39,100,000
Number of loans to be repurchased by providers									242								475
Salvage and subrogation recoverable
For all Financial Guaranty Insurance Contracts	401,800,000	866,200,000
Effect of Consolidating FG VIEs	(197,300,000)	(52,900,000)
Reported on Balance Sheet	204,500,000	813,300,000
Loss and LAE reserve
For all Financial Guaranty Insurance Contracts	857,500,000	490,600,000
Effect of Consolidating FG VIEs	(74,600,000)	(85,800,000)
Reported on Balance Sheet	782,900,000	404,800,000
Unearned premium reserve
For all Financial Guaranty Insurance Contracts	175,500,000	243,700,000
Effect of Consolidating FG VIEs	(49,900,000)	(22,500,000)
Reported on Balance Sheet	125,600,000	221,200,000
Total
Benefit for R&W	1,434,800,000	1,600,500,000	552,200,000		1,434,800,000	1,600,500,000	202,700,000	81,000,000		567,900,000	3,000,000	1,100,000	713,900,000	309,300,000	101,500,000	26,800,000		57,300,000					223,800,000	178,200,000	189,900,000	1,004,100,000
Effect of Consolidating FG VIEs	(321,800,000)	(161,200,000)
Reported on Balance Sheet	1,113,000,000	1,439,300,000
Rollforward of Estimated Benefit from Recoveries of Representation and Warranty Breaches, Net of Reinsurance
Future Net R&W Benefit at the beginning of the period	1,600,500,000		552,200,000		1,600,500,000	1,173,100,000	81,000,000	64,200,000			1,100,000		309,300,000	203,700,000	26,800,000								178,200,000	76,500,000	1,004,100,000	828,700,000
R&W Development and Accretion of Discount	889,400,000	616,500,000			889,400,000	616,500,000	122,800,000	16,800,000			1,900,000	1,100,000	496,100,000	166,600,000	74,700,000	26,800,000							54,600,000	101,700,000	139,300,000	303,500,000
R&W Recovered					(1,055,100,000)	(189,100,000)	(1,100,000)			(58,800,000)			(91,500,000)	(61,000,000)				(1,042,700,000)					(9,000,000)		(953,500,000)	(128,100,000)
Future Net R&W Benefit at the end of the period	1,434,800,000	1,600,500,000	552,200,000		1,434,800,000	1,600,500,000	202,700,000	81,000,000		567,900,000	3,000,000	1,100,000	713,900,000	309,300,000	101,500,000	26,800,000		57,300,000					223,800,000	178,200,000	189,900,000	1,004,100,000
Gross R&W amounts recovered					1,191,200,000	217,600,000
Number of Risks as of the end of the period with R&W Benefit Recorded (in policies)					59	46	22	17			1	1	12	10	5	1							4	4	15	13
Debt Service of Policies with R&W Benefit Recorded as of the end of the period					7,848,700,000	7,493,100,000	1,781,100,000	1,882,800,000			52,300,000	57,100,000	1,621,400,000	1,909,800,000	1,054,000,000	228,700,000							361,400,000	444,900,000	2,978,500,000	2,969,800,000
Breakdown of the development and accretion amount
Inclusion of new deals with breaches of R&W during period	108,200,000	170,500,000
Change in recovery assumptions as the result of additional file review and recovery success	218,300,000	253,500,000
Estimated increase (decrease) in defaults that will result in additional breaches	(77,100,000)	188,100,000
Results of settlements	622,000,000
Accretion of discount on balance	18,000,000	4,400,000
Total	889,400,000	616,500,000			889,400,000	616,500,000	122,800,000	16,800,000			1,900,000	1,100,000	496,100,000	166,600,000	74,700,000	26,800,000							54,600,000	101,700,000	139,300,000	303,500,000
Recovery period assumed in base case (in years)																			2 years	2 years	4 years	4 years
Number of additional transactions resulting in development		8
Additional information related to Representations and Warranties
Number of transactions insured										21								8
Number of transactions insured executed as financial guarantees										20
Cumulative collateral losses										1,900,000,000
Cumulative projected collateral losses										4,800,000,000
Gross expected losses before considering R&W recoveries										652,500,000
Gross expected losses after considering R&W recoveries										130,500,000
Payment reimbursed to date										58,800,000
Additional invoices for claims paid										6,700,000
Estimated benefit from loan repurchases relating breaches of R&W	$ 1,400,000,000