Debt (Debt Outstanding) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 2012 OBH Senior Notes	Nov. 30, 2012 2012 OBH Senior Notes	Dec. 31, 2011 2012 OBH Senior Notes	Dec. 31, 2012 2003 OBH Senior Notes	Dec. 31, 2011 2003 OBH Senior Notes	Dec. 31, 2012 SIG Senior Notes	Dec. 31, 2011 SIG Senior Notes	Mar. 31, 2007 SIG Senior Notes	Dec. 31, 2012 WTM Bank Facility	Dec. 31, 2011 WTM Bank Facility	Dec. 31, 2012 Old Lyme	Dec. 31, 2011 Old Lyme	Dec. 31, 2012 Hamer and Bri-Mar		Dec. 31, 2011 Hamer and Bri-Mar
Debt Instrument
Debt instrument at face value			$ 275.0	$ 275.0	$ 0	$ 0	$ 269.9	$ 400.0	$ 400.0	$ 400.0
Unamortized original issue discount			(0.3)		0	0	(0.1)	(0.6)	(0.7)
Debt	$ 751.2	$ 677.5	$ 274.7		$ 0	$ 0	$ 269.8	$ 399.4	$ 399.3		$ 75.0	$ 0	$ 2.1	$ 2.1	$ 0	[1]	$ 6.3	[1]

[1] Other debt represents debt of Hamer and Bri-Mar, which are no longer consolidated as of October 1, 2012. See Note 16.