Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Share-based compensation arrangement by share-based payment award
Total Accrued Expense	$ 63.3		$ 95.7
White Mountains | Performance Share | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	51,345
Accrued Expense	$ 31.7
Target Performance Shares Outstanding, Assumed forfeitures	(770)
Accrued Expense, Assumed forfeitures	(0.5)
Total Target Performance Shares Outstanding	50,575	56,805	80,353	73,297	108,683	93,654
Total Accrued Expense	$ 31.2	$ 24.0	$ 42.4	$ 32.2	$ 71.3	$ 57.7
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	18,470
Accrued Expense	$ 18.3
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2016 to 2018
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	16,315
Accrued Expense	$ 10.3
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2017 to 2019
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	16,560
Accrued Expense	$ 3.1