Business and Basis of Presentation (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012 Company Subsidiary Security	Dec. 31, 2011	May 31, 2012 MIAC jurisdictions
Credit facilities
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
Percent of outstanding common stock purchased			100.00%
Cash paid to acquire entity			$ 91
Allocation of purchase price to intangible assets other than goodwill			16
Number of jurisdictions in which entity is licensed to operate			38
Credit DerivativeAsset Liability Reclassification		$ 316