Financial Guaranty Insurance Contracts (Details 8) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011 entity security segment agreement market subsidiary company policy D transaction counterparty	Sep. 30, 2010	Sep. 30, 2011 subsidiary statement ratingcategory security Y segment entity agreement market company policy D transaction counterparty	Sep. 30, 2010	Dec. 31, 2010 entity security policy
Financial guarantee obligations
Number of negotiated agreements	2		2
Threshold days delinquent for which all loan files are requested for review (in days)	60		60
Salvage and subrogation recoverable
For all Financial Guaranty Insurance Contracts	$ 386,300,000		$ 386,300,000		$ 866,200,000
Effect of Consolidating FG VIEs	(200,900,000)		(200,900,000)		(52,900,000)
Reported on Balance Sheet	185,400,000		185,400,000		813,300,000
Loss and LAE reserve
For all Financial Guaranty Insurance Contracts	978,100,000		978,100,000		490,600,000
Effect of Consolidating FG VIEs	(89,600,000)		(89,600,000)		(85,800,000)
Reported on Balance Sheet	888,500,000		888,500,000		404,800,000
Unearned premium reserve
For all Financial Guaranty Insurance Contracts	186,600,000		186,600,000		243,700,000
Effect of Consolidating FG VIEs	(43,700,000)		(43,700,000)		(22,500,000)
Reported on Balance Sheet	142,900,000		142,900,000		221,200,000
Total
For all Financial Guaranty Insurance Contracts	1,551,000,000	1,306,900,000	1,551,000,000	1,306,900,000
Effect of Consolidating FG VIEs	(334,200,000)		(334,200,000)		(161,200,000)
Reported on Balance Sheet	1,216,800,000		1,216,800,000		1,439,300,000
Roll forward 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	1,173,100,000
R&W Development and Accretion of Discount	159,100,000	53,300,000	930,000,000	265,200,000
R&W Recovered			(979,500,000)	(131,400,000)
Future Net R&W Benefit at the end of the period	1,551,000,000	1,306,900,000	1,551,000,000	1,306,900,000
R&W benefit attributable to transactions covered by the Bank of America Agreement	649,800,000		649,800,000
Number of Risks as of the end of the period with R&W Benefit Recorded	56		56		46
Outstanding Principal and Interest of Policies with R&W Benefit Recorded as of the end of the period	8,572,800,000		8,572,800,000		7,493,100,000
Breakdown of the development and accretion amount
Inclusion of new deals with breaches of R&W during period	1,100,000		108,200,000	62,400,000
Change in recovery assumptions as the result of additional file review and recovery success	43,300,000	(3,400,000)	241,700,000	61,900,000
Estimated increase (decrease) in defaults that will result in additional (lower) breaches	(22,600,000)	50,100,000	11,400,000	132,200,000
Results of Bank of America Agreement	129,800,000		559,500,000
Accretion of discount on balance	7,500,000	6,600,000	9,200,000	8,700,000
Total	159,100,000	53,300,000	930,000,000	265,200,000
Percent of recoveries for first lien transactions under the Bank of America Agreement, less sharing agreement	80.00%		80.00%
U.S. RMBS
Roll forward of Estimated Benefit from Recoveries of Representation and Warranty Breaches, Net of Reinsurance
Gross R&W amounts recovered			1,107,800,000	154,400,000
Alt-A first lien
Total
For all Financial Guaranty Insurance Contracts	192,700,000	84,000,000	192,700,000	84,000,000
Roll forward of Estimated Benefit from Recoveries of Representation and Warranty Breaches, Net of Reinsurance
Future Net R&W Benefit at the beginning of the period			81,000,000	64,200,000
R&W Development and Accretion of Discount			111,700,000	19,800,000
Future Net R&W Benefit at the end of the period	192,700,000	84,000,000	192,700,000	84,000,000
Number of Risks as of the end of the period with R&W Benefit Recorded	20		20		17
Outstanding Principal and Interest of Policies with R&W Benefit Recorded as of the end of the period	1,778,400,000		1,778,400,000		1,882,800,000
Breakdown of the development and accretion amount
Total			111,700,000	19,800,000
Total first lien
Financial guarantee obligations
Number of defaulted loan files reviewed	17,000		17,000
Outstanding par of defaulted loans reviewed	4,941,000		4,941,000
Number of loan files that breached one or more R&W	15,500		15,500
Amount of loans to be repurchased by providers	54,000,000		54,000,000
Number of loans to be repurchased by providers	167		167
Breakdown of the development and accretion amount
Number of transactions insured			21
Cumulative collateral losses			1,800,000,000
Cumulative projected collateral losses			4,900,000,000
Gross expected losses before considering R&W recoveries from Bank of America			754,200,000
Gross expected losses after considering R&W recoveries from Bank of America			150,800,000
Payment reimbursed by Bank of America			34,100,000
Prime first lien
Total
For all Financial Guaranty Insurance Contracts	3,000,000	1,000,000	3,000,000	1,000,000
Roll forward of Estimated Benefit from Recoveries of Representation and Warranty Breaches, Net of Reinsurance
Future Net R&W Benefit at the beginning of the period			1,100,000
R&W Development and Accretion of Discount			1,900,000	1,000,000
Future Net R&W Benefit at the end of the period	3,000,000	1,000,000	3,000,000	1,000,000
Number of Risks as of the end of the period with R&W Benefit Recorded	1		1		1
Outstanding Principal and Interest of Policies with R&W Benefit Recorded as of the end of the period	54,000,000		54,000,000		57,100,000
Breakdown of the development and accretion amount
Total			1,900,000	1,000,000
Option ARM
Total
For all Financial Guaranty Insurance Contracts	772,700,000	248,000,000	772,700,000	248,000,000
Roll forward of Estimated Benefit from Recoveries of Representation and Warranty Breaches, Net of Reinsurance
Future Net R&W Benefit at the beginning of the period			309,300,000	203,700,000
R&W Development and Accretion of Discount			530,900,000	86,800,000
R&W Recovered			(67,500,000)	(42,500,000)
Future Net R&W Benefit at the end of the period	772,700,000	248,000,000	772,700,000	248,000,000
Number of Risks as of the end of the period with R&W Benefit Recorded	11		11		10
Outstanding Principal and Interest of Policies with R&W Benefit Recorded as of the end of the period	1,768,000,000		1,768,000,000		1,909,800,000
Breakdown of the development and accretion amount
Total			530,900,000	86,800,000
Subprime
Total
For all Financial Guaranty Insurance Contracts	107,700,000		107,700,000
Roll forward of Estimated Benefit from Recoveries of Representation and Warranty Breaches, Net of Reinsurance
Future Net R&W Benefit at the beginning of the period			26,800,000
R&W Development and Accretion of Discount			80,900,000
Future Net R&W Benefit at the end of the period	107,700,000		107,700,000
Number of Risks as of the end of the period with R&W Benefit Recorded	5		5		1
Outstanding Principal and Interest of Policies with R&W Benefit Recorded as of the end of the period	1,071,700,000		1,071,700,000		228,700,000
Breakdown of the development and accretion amount
Total			80,900,000
Second lien
Financial guarantee obligations
Number of defaulted loan files reviewed	15,800		15,800
Outstanding par of defaulted loans reviewed	1,089,000		1,089,000
Number of loan files that breached one or more R&W	15,000		15,000
Amount of loans to be repurchased by providers	39,000,000		39,000,000
Number of loans to be repurchased by providers	475		475
Breakdown of the development and accretion amount
Number of transactions insured			8
Closed end second lien and HELOCs
Breakdown of the development and accretion amount
Recovery period, low end of range (in years)	2		2
Recovery period, high end of range (in years)	4		4
CES
Total
For all Financial Guaranty Insurance Contracts	206,800,000	136,000,000	206,800,000	136,000,000
Roll forward of Estimated Benefit from Recoveries of Representation and Warranty Breaches, Net of Reinsurance
Future Net R&W Benefit at the beginning of the period			178,200,000	76,500,000
R&W Development and Accretion of Discount			37,600,000	59,500,000
R&W Recovered			(9,000,000)
Future Net R&W Benefit at the end of the period	206,800,000	136,000,000	206,800,000	136,000,000
Number of Risks as of the end of the period with R&W Benefit Recorded	4		4		4
Outstanding Principal and Interest of Policies with R&W Benefit Recorded as of the end of the period	378,700,000		378,700,000		444,900,000
Breakdown of the development and accretion amount
Total			37,600,000	59,500,000
HELOCs
Total
For all Financial Guaranty Insurance Contracts	268,100,000	837,900,000	268,100,000	837,900,000
Roll forward of Estimated Benefit from Recoveries of Representation and Warranty Breaches, Net of Reinsurance
Future Net R&W Benefit at the beginning of the period			1,004,100,000	828,700,000
R&W Development and Accretion of Discount			167,000,000	98,100,000
R&W Recovered			(903,000,000)	(88,900,000)
Future Net R&W Benefit at the end of the period	268,100,000	837,900,000	268,100,000	837,900,000
Number of Risks as of the end of the period with R&W Benefit Recorded	15		15		13
Outstanding Principal and Interest of Policies with R&W Benefit Recorded as of the end of the period	3,522,000,000		3,522,000,000		2,969,800,000
Breakdown of the development and accretion amount
Total			$ 167,000,000	$ 98,100,000