Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013
Share-based compensation arrangement by share-based payment award
Total Accrued Expense	$ 124.9		$ 184.6
White Mountains | Performance Share | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	120,130
Accrued Expense	$ 31.9
Target Performance Shares Outstanding, Assumed forfeitures	(3,003)
Accrued Expense, Assumed forfeitures	(0.8)
Total Target Performance Shares Outstanding	117,127	116,396	123,549	127,537	127,537	119,220
Total Accrued Expense	$ 31.1	$ 27.6	$ 48.1	$ 50.5	$ 42.3	$ 60.2
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2015 to 2017
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	31,390
Accrued Expense	$ 3.4
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2014 to 2016
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	41,937
Accrued Expense	$ 5.1
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2013 to 2015
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	46,803
Accrued Expense	$ 23.4