Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) (White Mountains, W T M Incentive Plan [Member], USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 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 [Member]
Target Restricted Share Activity
Issued (in shares)	25,470
Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	94,130	69,910
Issued (in shares)	20,400	25,470
Vested (in shares)	(33,205)	0
Assumed forfeitures and cancellations	0	0
Ending of period (in shares)	81,325	95,380
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 17.0	$ 16.8
Issued	11.8	14.3
Assumed forfeitures and cancellations	0	0
Expense recognized	(4.0)	(2.9)
Unamortized grant date fair value, ending balance	$ 24.8	$ 28.2