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		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Restricted Share Cliff Vesting in January 2017 [Member]
Target Restricted Share Activity
Issued (in shares)			20,400
Restricted Share Cliff Vesting in January 2016
Target Restricted Share Activity
Issued (in shares)			25,720
Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	81,325	95,380	94,130	69,910
Issued (in shares)	0	250	20,400	25,720
Vested (in shares)	0	0	(33,205)	0
Assumed forfeitures and cancellations	0	0	0	0
Ending of period (in shares)	81,325	95,630	81,325	95,630
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 20.9	$ 24.5	$ 17.0	$ 16.8
Issued	0	0.1	11.8	14.4
Assumed forfeitures and cancellations	0	0	0	0
Expense recognized	(3.3)	(3.8)	(11.2)	(10.4)
Unamortized grant date fair value, ending balance	$ 17.6	$ 20.8	$ 17.6	$ 20.8