Debt (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended							1 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 2012 OBH Senior Notes	Dec. 31, 2012 2012 OBH Senior Notes	Jun. 30, 2013 SIG Senior Notes	Dec. 31, 2012 SIG Senior Notes	Dec. 31, 2012 WTM Bank Facility	Jun. 30, 2013 WTM Bank Facility	Dec. 31, 2012 WTM Bank Facility	Jan. 31, 2013 WTM Bank Facility	Aug. 12, 2011 WTM Bank Facility	Jun. 30, 2013 Old Lyme	Dec. 31, 2012 Old Lyme
Debt Instrument
Debt instrument at face value				$ 275.0	$ 275.0	$ 400.0	$ 400.0
Unamortized original issue discount				(0.3)	(0.3)	(0.5)	(0.6)
Total debt	676.3		751.2	274.7	274.7	399.5	399.4	75.0	0	75.0	75.0		2.1	2.1
Total commitment under revolving credit facility												375.0
Proceeds from lines of credit	130.0	0							130.0	150.0
Repayments of debt	$ 205.0	$ 0						$ 75.0	$ 130.0
Debt, Weighted Average Interest Rate									2.71%