Financial Guaranty Insurance Contracts (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Effect of Retrospective Application of New Deferred Acquisition Cost Guidance
Deferred acquisition costs	$ (126,755)		$ (126,755)		$ (132,418)
Retained earnings	1,568,397		1,568,397		1,707,922
Amortization of deferred acquisition costs	4,526	5,810	9,939	9,472
Other operating expenses	53,447	53,249	114,727	116,132
Net income (loss)	376,548	(43,105)	(106,476)	96,151
Earnings Per Share [Abstract]
Basic (in dollars per share)	$ 2.02	$ (0.23)	$ (0.58)	$ 0.52
Diluted (in dollars per share)	$ 2.01	$ (0.23)	$ (0.58)	$ 0.51
As Reported
Effect of Retrospective Application of New Deferred Acquisition Cost Guidance
Amortization of deferred acquisition costs		9,500		16,900
Other operating expenses		48,500		105,300
Net income (loss)		(42,600)		98,000
Earnings Per Share [Abstract]
Basic (in dollars per share)		$ (0.23)		$ 0.53
Diluted (in dollars per share)		$ (0.23)		$ 0.52
Retroactive Application Adjustment
Effect of Retrospective Application of New Deferred Acquisition Cost Guidance
Amortization of deferred acquisition costs		(3,700)		(7,400)
Other operating expenses		4,700		10,700
Net income (loss)		(500)		(1,800)
Earnings Per Share [Abstract]
Basic (in dollars per share)		$ 0.00		$ (0.01)
Diluted (in dollars per share)		$ 0.00		$ (0.01)
Retrospective Application of New Deferred Acquisition Cost Guidance | Retroactive Application Adjustment
Effect of Retrospective Application of New Deferred Acquisition Cost Guidance
Deferred acquisition costs					94,400
Retained earnings					$ 64,000