Financial Guaranty Insurance Contracts (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended			3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2011 As Reported	Sep. 30, 2011 As Reported	Sep. 30, 2011 Retroactive Application Adjustment	Sep. 30, 2011 Retroactive Application Adjustment	Dec. 31, 2010 Retrospective Application of New Deferred Acquisition Cost Guidance Retroactive Application Adjustment
Effect of Retrospective Application of New Deferred Acquisition Cost Guidance
Financial Guaranty Insurance Contracts, Installment Premium Receivable Percentage Foreign Denominated	50.00%	49.00%	50.00%	49.00%	47.00%
Deferred acquisition costs	$ (127)		$ (127)		$ (132)					$ 94
Retained earnings	1,693		1,693		1,708					64
Amortization of deferred acquisition costs	4	4	14	13		7	24	(3)	(11)
Other operating expenses	48	46	163	163		42	147	4	16
Net income (loss)	$ 142	$ 761	$ 36	$ 857		$ 761	$ 859	$ 0	$ (2)
Earnings Per Share [Abstract]
Basic (in dollars per share)	$ 0.73	$ 4.15	$ 0.19	$ 4.66		$ 4.15	$ 4.67	$ 0.00	$ (0.01)
Diluted (in dollars per share)	$ 0.73	$ 4.13	$ 0.19	$ 4.60		$ 4.13	$ 4.61	$ 0.00	$ (0.01)