Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) (White Mountains, W T M Incentive Plan, USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Restricted Share Cliff Vesting in January 2018
Target Restricted Share Activity
Issued (in shares)	22,890
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)	83,314	94,130
Issued (in shares)	22,890	20,400
Vested (in shares)	(35,079)	(33,205)
Assumed forfeitures and cancellations	0	0
Ending of period (in shares)	71,125	81,325
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 14.3	$ 17.0
Issued	15.2	11.8
Assumed forfeitures and cancellations	0	0
Expense recognized	(3.0)	(4.0)
Unamortized grant date fair value, ending balance	$ 26.5	$ 24.8