Business and Basis of Presentation (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012 reinsurer Company Subsidiary Security
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
Amount of previously ceded business reassumed	$ 192.1
Number of reinsurers from whom the company has reassumed preciously ceded business	2
Number of UK insurance subsidiaries	2
Number of holding companies having public debt outstanding	2
AGM and AGC
Credit facilities
Losses Assumed by Reinsurers under Excess of Loss Reinsurance Facility	435
Excess of loss reinsurance facility
Credit facilities
Amount of previously ceded business reassumed	$ 19,100.0