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	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	41,220
Accrued Expense	$ 23.4
Target Performance Shares Outstanding, Assumed forfeitures	(618)
Accrued Expense, Assumed forfeitures	(0.3)
Total Target Performance Shares Outstanding	40,602	40,616	40,616	50,515
Total Accrued Expense	$ 23.1	$ 31.7	$ 21.0	$ 45.8
Performance Cycle 2017 to 2019
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	14,070
Accrued Expense	$ 16.9
Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	13,450
Accrued Expense	$ 5.5
Performance Cycle 2019 to 2021
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	13,700
Accrued Expense	$ 1.0