Business Changes, Risks, Uncertainties and Accounting Developments (Details 2) (USD $)				12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Bank of America agreement	Dec. 31, 2011 RMBS Bank of America agreement Transaction	Dec. 31, 2011 Total first lien Bank of America agreement Transaction	Dec. 31, 2011 Second lien Bank of America agreement Transaction
Financial guarantee obligations
Number of transactions insured				29	21	8
Gross Par Outstanding	$ 614,342,000,000	$ 681,248,000,000		$ 4,400,000,000
Net Par Outstanding	558,048,000,000	617,131,000,000		4,100,000,000
Net par outstanding as percent of total below-investment-grade ("BIG") net par outstanding				27.00%
Recovery payments received					58,800,000	1,042,700,000
Additional invoices for claims paid					6,700,000
Recovery obligations to be received	1,434,800,000	1,600,500,000	552,200,000		567,900,000	57,300,000
Total consideration under Bank of America agreement						1,100,000,000
Loss sharing agreement (as a percent)					80.00%
Maximum aggregate collateral losses					6,600,000,000
Collateral losses					2,000,000,000
Eligible assets in trust for payment of reimbursement					$ 941,200,000