Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) - White Mountains - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Restricted Share Cliff Vesting in January 2018
Target Restricted Share Activity
Issued (in shares)			23,640
Restricted Share Cliff Vesting in January 2017
Target Restricted Share Activity
Issued (in shares)			20,400
Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	71,125	81,325	83,314	94,130	94,130
Issued (in shares)	750	0	23,640	20,400
Vested (in shares)	0	0	(35,079)	(33,205)
Assumed forfeitures and cancellations	0	0	0	0
Ending of period (in shares)	71,875	81,325	71,875	81,325	83,314
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 26.5	$ 24.8	$ 14.3	$ 17.0	$ 17.0
Issued	0.5	0.0	15.7	11.8
Assumed forfeitures and cancellations	0.0	0.0	0.0	0.0
Expense recognized	(3.8)	(3.9)	(6.8)	(7.9)
Unamortized grant date fair value, ending balance	$ 23.2	$ 20.9	$ 23.2	$ 20.9	$ 14.3