Financial Guaranty Insurance Contracts (Details 5) (USD $)	12 Months Ended
	Dec. 31, 2011 Payment	Dec. 31, 2010	Dec. 31, 2009
Reconciliation of Expected Loss to be Paid and Net Expected Loss to be Expensed
Salvage and subrogation recoverable	$ 367,718,000	$ 1,032,369,000
Ceded salvage and subrogation recoverable	(40,600,000)	(129,400,000)
Loss and LAE reserve	(609,700,000)	(552,100,000)
Reinsurance recoverable on unpaid losses	69,300,000	22,255,000
Net expected loss to be expensed related to consolidated financial guaranty VIEs	222,700,000	211,900,000
Maximum number of payments behind to be considered performing borrower	1
Loss severity (as a percent)		80.00%	70.00%
Minimum
Reconciliation of Expected Loss to be Paid and Net Expected Loss to be Expensed
Peak loss severities (as a percent)	60.00%
Maximum
Reconciliation of Expected Loss to be Paid and Net Expected Loss to be Expensed
Peak loss severities (as a percent)	75.00%
Total first lien
Reconciliation of Expected Loss to be Paid and Net Expected Loss to be Expensed
Number of transactions for which credits were added in loss projection methodology for recoveries on R&W, where loan files were obtained		4
Second lien
Reconciliation of Expected Loss to be Paid and Net Expected Loss to be Expensed
Loss severity (as a percent)		98.00%	95.00%
Number of transactions for which credits were added in loss projection methodology for recoveries on R&W, where loan files were obtained		2
Number of transactions for which credits were added in loss projection methodology for recoveries on R&W		4
Number of transactions for which credits were added in loss projection methodology for recoveries on R&W, where a capital infusion was received		2
Number of types of RMBS insured	2
Subprime
Reconciliation of Expected Loss to be Paid and Net Expected Loss to be Expensed
Loss severity (as a percent)	90.00%	80.00%	70.00%
Subprime | Minimum
Reconciliation of Expected Loss to be Paid and Net Expected Loss to be Expensed
Loss severity (as a percent)	80.00%
Subprime | Maximum
Reconciliation of Expected Loss to be Paid and Net Expected Loss to be Expensed
Loss severity (as a percent)	90.00%
Other first lien | Minimum
Reconciliation of Expected Loss to be Paid and Net Expected Loss to be Expensed
Loss severity (as a percent)	60.00%
Other first lien | Maximum
Reconciliation of Expected Loss to be Paid and Net Expected Loss to be Expensed
Loss severity (as a percent)	65.00%
Financial Guaranty
Reconciliation of Expected Loss to be Paid and Net Expected Loss to be Expensed
Net expected loss to be paid	638,000,000	408,800,000	1,066,400,000
Less: net expected loss to be paid for FG VIEs	106,700,000	(49,200,000)
Total	744,700,000	359,600,000
Contra-paid, net	73,400,000	121,300,000
Salvage and subrogation recoverable	367,718,000	1,032,369,000
Ceded salvage and subrogation recoverable	(40,600,000)	(129,400,000)
Loss and LAE reserve	(676,900,000)	(570,800,000)
Reinsurance recoverable on unpaid losses	69,100,000	20,800,000
Net expected loss to be expensed	537,400,000	833,900,000
Financial Guaranty | Total first lien
Reconciliation of Expected Loss to be Paid and Net Expected Loss to be Expensed
Net expected loss to be paid	429,900,000	909,900,000	827,100,000
Financial Guaranty | Second lien
Reconciliation of Expected Loss to be Paid and Net Expected Loss to be Expensed
Net expected loss to be paid	(110,700,000)	(749,100,000)	(7,300,000)
Financial Guaranty | Subprime
Reconciliation of Expected Loss to be Paid and Net Expected Loss to be Expensed
Net expected loss to be paid	140,300,000	200,400,000	77,500,000
Other
Reconciliation of Expected Loss to be Paid and Net Expected Loss to be Expensed
Net expected loss to be paid	$ 1,900,000	$ 2,100,000