Business and Basis of Presentation (Details)	9 Months Ended
	Sep. 30, 2011 subsidiary statement ratingcategory security Y segment entity agreement market company policy D transaction counterparty
Business and Basis of Presentation
Financial guaranty contracts accounted for as credit derivatives, minimum number of credit events triggering obligations	1
Financial guaranty contracts accounted for as credit derivatives, minimum number of third party referenced securities or loans	1
Number of UK insurance subsidiaries	2
Number of holding companies having public debt outstanding	2
Number of separate but consecutive statements	2
Number of business segments prior to January 2011	2
Period of no market for financial guaranty reinsurance (in years)	2
Number of markets for financial guaranty reinsurance that are not expected to develop in the foreseeable future	1