Employee Share-Based Incentive Compensation Plans (Performance Shares Granted Under the WTM Incentive Plan) (Details) - White Mountains - Performance Share - W T M Incentive Plan - USD ($) $ in Millions	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	41,235
Accrued Expense	$ 27.6
Target Performance Shares Outstanding, Assumed forfeitures	(619)
Accrued Expense, Assumed forfeitures	(0.4)
Total Target Performance Shares Outstanding	40,616	40,616	50,515	50,575	56,805	80,353
Total Accrued Expense	$ 27.2	$ 21.0	$ 45.8	$ 31.2	$ 24.0	$ 42.4
Performance Cycle 2016 to 2018
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	13,715
Accrued Expense	$ 15.4
Performance Cycle 2017 to 2019
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	14,070
Accrued Expense	$ 10.1
Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	13,450
Accrued Expense	$ 2.1