Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Share-based compensation arrangement by share-based payment award
Total Accrued Expense	$ 76.9		$ 95.7
White Mountains | Performance Share | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	51,085
Accrued Expense	$ 38.4
Target Performance Shares Outstanding, Assumed forfeitures	(766)
Accrued Expense, Assumed forfeitures	(0.6)
Total Target Performance Shares Outstanding	50,319	50,575	80,353	79,537	73,297	93,654
Total Accrued Expense	$ 37.8	$ 31.2	$ 42.4	$ 37.4	$ 32.5	$ 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	18,370
Accrued Expense	$ 20.6
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	16,235
Accrued Expense	$ 12.1
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	16,480
Accrued Expense	$ 5.7