Debt (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended							1 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 2012 OBH Senior Notes	Dec. 31, 2012 2012 OBH Senior Notes	Sep. 30, 2013 SIG Senior Notes	Dec. 31, 2012 SIG Senior Notes	Dec. 31, 2012 WTM Bank Facility	Sep. 30, 2013 WTM Bank Facility	Dec. 31, 2012 WTM Bank Facility	Jan. 31, 2013 WTM Bank Facility	Aug. 12, 2011 WTM Bank Facility	Sep. 30, 2013 Prior W T M Bank Facility [Member]	Dec. 31, 2012 Prior W T M Bank Facility [Member]	Aug. 12, 2011 Prior W T M Bank Facility [Member]	Sep. 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	0	0	0	75.0		0	75.0		2.1	2.1
Total commitment under revolving credit facility												425.0			375.0
Proceeds from lines of credit	150.0	0							150.0	150.0
Repayments of debt	$ 225.0	$ 0						$ 75.0	$ 130.0
Debt, Weighted Average Interest Rate									2.83%