Financial Guaranty Insurance Contracts (Details 14) (USD $)	12 Months Ended
	Dec. 31, 2011 Subsidiary RatingCategory	Dec. 31, 2010	Dec. 31, 2009
Components of Net Reserves (Salvage)
Loss and loss adjustment expense reserve	$ 679,011,000	$ 574,369,000	$ 306,400,000
Reinsurance recoverable on unpaid losses	(69,300,000)	(22,255,000)
Salvage and subrogation recoverable	(367,718,000)	(1,032,369,000)
Salvage and subrogation payable	40,600,000	129,400,000
Total	282,600,000	(350,900,000)
Number of the entity's insurance subsidiaries that results in increased claims due to downgrade	1
Minimum number of rating agency considered for triggering bank termination right	1
Minimum
Components of Net Reserves (Salvage)
Threshold period of bonds held by bank for right of accelerated repayment (in days)	180 days
Accelerated repayment period for bank bonds (in years)	10 years
Variable rate demand obligations
Components of Net Reserves (Salvage)
Rate basis for bank bond rate	prime rate
Bank bond floor rate (as a percent)	7.00%
Par of insured variable rate demand obligations issued by municipal obligors with internal ratings of BBB- or lower	1,100,000,000
Variable rate demand obligations | Minimum
Components of Net Reserves (Salvage)
Bank bond rate (as a percent)	2.00%
Threshold period of bonds held by bank for right of accelerated repayment (in days)	90 days
Accelerated repayment period for bank bonds (in years)	5 years
Variable rate demand obligations | Maximum
Components of Net Reserves (Salvage)
Bank bond rate (as a percent)	3.00%
Financial Guaranty
Components of Net Reserves (Salvage)
Reinsurance recoverable on unpaid losses	(69,100,000)	(20,800,000)
Salvage and subrogation recoverable	(367,718,000)	(1,032,369,000)
Salvage and subrogation payable	40,600,000	129,400,000
Total	280,700,000	(353,000,000)
Other
Components of Net Reserves (Salvage)
Total	$ 1,900,000	$ 2,100,000