Debt and Standby Letter of Credit Facilities (Debt Outstanding) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Aug. 14, 2013
Debt Instrument [Line Items]
Debt	$ 285.9	$ 337.6
Interest expense on debt	16.1	14.6	$ 14.2
Draw down of debt and revolving line of credit	352.5	171.5	133.6
WTM Bank Facility
Debt Instrument [Line Items]
Total commitment under revolving credit facility				$ 425.0
Interest expense on debt	1.2	0.1	$ 0.3
Prior W T M Bank Facility
Debt Instrument [Line Items]
Total commitment under revolving credit facility				$ 375.0
OneBeacon | 2012 OBH Senior Notes
Debt Instrument [Line Items]
Debt instrument at face value	275.0	275.0
Unamortized original issue discount	(1.8)	(2.1)
Debt	$ 273.2	$ 272.9
Effective yield (as a percent)	4.70%	4.70%
White Mountains | WTM Bank Facility
Debt Instrument [Line Items]
Debt	$ 0.0	$ 50.0
Effective yield (as a percent)		3.90%
MediaAlpha [Member] | MediaAlpha Bank Facility [Member]
Debt Instrument [Line Items]
Total commitment under revolving credit facility		$ 20.0
Debt instrument at face value	12.9	15.0
Unamortized Debt Issuance Expense	$ (0.2)	$ (0.3)
Effective yield (as a percent)	5.70%	5.50%
Carrying Value | OneBeacon | 2012 OBH Senior Notes
Debt Instrument [Line Items]
Debt	$ 273.2	$ 272.9
Carrying Value | MediaAlpha [Member] | MediaAlpha Bank Facility [Member]
Debt Instrument [Line Items]
Debt	$ 12.7	$ 14.7