Financial Guaranty Contracts Accounted for as Insurance (Details 9) (USD $) In Millions, unless otherwise specified													6 Months Ended
	Jun. 30, 2011 Securities Policies D Companies Agreements Markets Subsidiaries Transactions FiscalQuarters Counterparties Entities	Dec. 31, 2010	Jun. 30, 2011 "XXX" Life Insurance Transactions Transactions	Jun. 30, 2011 "XXX" Life Insurance Transactions Below investment grade ("BIG")	Jun. 30, 2011 Public finance Below investment grade ("BIG")	Jun. 30, 2011 Student Loan	Jun. 30, 2011 Student loan.	Jun. 30, 2011 Student loan. Below investment grade ("BIG")	Jun. 30, 2011 Asset-backed student loan	Jun. 30, 2011 Asset-backed student loan Below investment grade ("BIG")	Jun. 30, 2011 Jefferson County Alabama Sewer Authority	Jun. 30, 2011 Telephone directory "yellow pages"	Jun. 30, 2011 Manufactured housing loans Transactions	Jun. 30, 2011 Manufactured housing loans Below investment grade ("BIG")	Jun. 30, 2011 Harrisburg, Pennsylvania	Jun. 30, 2011 Harrisburg, Pennsylvania Below investment grade ("BIG")	Jun. 30, 2011 Second lien Securities Loans Transactions D
Financial guarantee obligations
Net Par Outstanding Financial Guaranty Insurance			$ 2,000.0	$ 882.5	$ 531.9	$ 2,900.0	$ 1,400.0		$ 1,500.0	$ 242.3	$ 495.8	$ 110.7	$ 67.1	$ 240.5	$ 165.0	$ 93.1
Largest expected loss to be paid for a single transaction			72.9			32.8		12.6		60.7	31.1	6.5	14.5
Loss and LAE reserve, net	492.1	540.7	61.2
Number of insured transactions			2
Number of transactions for which estimated loss was projected													1
Number of transactions backed by manufactured housing loans													2,000
Amount of transactions insured													358.8
Net claims to date															$ 4.5
Number of U.S. RMBS transactions for which lawsuits were filed alleging breaches of R&W	6
Number of U.S. RMBS transactions for which lawsuits were filed alleging violations of state securities laws and other claims	2																6