Business and Basis of Presentation (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 Statement Company Subsidiary Security segment	Dec. 31, 2010 segment	Jan. 02, 2012 Excess of loss reinsurance facility
Credit facilities
Maximum credit facility			$ 435.0
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