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		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 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)			25,470
Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	81,325	95,380	94,130	69,910
Issued (in shares)	0	0	20,400	25,470
Vested (in shares)	0	0	(33,205)	0
Assumed forfeitures and cancellations	0	0	0	0
Ending of period (in shares)	81,325	95,380	81,325	95,380
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 24.8	$ 28.2	$ 17.0	$ 16.8
Issued	0	0	11.8	14.3
Assumed forfeitures and cancellations	0	0	0	0
Expense recognized	(3.9)	(3.7)	(7.9)	(6.6)
Unamortized grant date fair value, ending balance	$ 20.9	$ 24.5	$ 20.9	$ 24.5