Debt (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Aug. 14, 2013 WTM Bank Facility	Sep. 30, 2014 Old Lyme	Dec. 31, 2013 Old Lyme	Sep. 30, 2014 OneBeacon 2012 OBH Senior Notes	Dec. 31, 2013 OneBeacon 2012 OBH Senior Notes	Sep. 30, 2014 Sirius Group SIG Senior Notes	Dec. 31, 2013 Sirius Group SIG Senior Notes	Sep. 30, 2014 White Mountains WTM Bank Facility	Dec. 31, 2013 White Mountains WTM Bank Facility	Sep. 30, 2014 QuoteLab	Dec. 31, 2013 QuoteLab
Debt Instrument
Debt instrument at face value						$ 275.0	$ 275.0	$ 400.0	$ 400.0
Unamortized original issue discount						(0.3)	(0.3)	(0.4)	(0.4)
Total debt	678.1	676.4		2.8	2.1	274.7	274.7	399.6	399.6	0	0	1.0	0
Total commitment under revolving credit facility			$ 425.0