Debt (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Aug. 14, 2013 WTM Bank Facility	Jun. 30, 2014 Old Lyme	Dec. 31, 2013 Old Lyme	Jun. 30, 2014 Other	Dec. 31, 2013 Other	Jun. 30, 2014 OneBeacon 2012 OBH Senior Notes	Dec. 31, 2013 OneBeacon 2012 OBH Senior Notes	Jun. 30, 2014 Sirius Group SIG Senior Notes	Dec. 31, 2013 Sirius Group SIG Senior Notes	Jun. 30, 2014 White Mountains WTM Bank Facility	Dec. 31, 2013 White Mountains WTM Bank Facility
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	677.5		676.4		2.1	2.1	1.1	0	274.7	274.7	399.6	399.6	0	0
Total commitment under revolving credit facility				425.0
Repayments of debt	$ 0.2	$ 205.0