Debt (Debt Outstanding) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 2012 OBH Senior Notes	Dec. 31, 2012 2012 OBH Senior Notes	Dec. 31, 2013 SIG Senior Notes	Dec. 31, 2012 SIG Senior Notes	Mar. 31, 2007 SIG Senior Notes	Dec. 31, 2013 WTM Bank Facility	Dec. 31, 2012 WTM Bank Facility	Dec. 31, 2013 Old Lyme	Dec. 31, 2012 Old Lyme	Dec. 31, 2011 Old Lyme	Dec. 31, 2013 Prior W T M Bank Facility	Jan. 31, 2013 Prior W T M Bank Facility	Dec. 31, 2012 Prior W T M Bank Facility
Debt Instrument
Debt instrument at face value			$ 275.0	$ 275.0	$ 400.0	$ 400.0	$ 400.0
Unamortized original issue discount			(0.3)	(0.3)	(0.4)	(0.6)
Debt	$ 676.4	$ 751.2	$ 274.7	$ 274.7	$ 399.6	$ 399.4		$ 0	$ 0	$ 2.1	$ 2.1	$ 2.1	$ 0	$ 75.0	$ 75.0