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	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	43,120
Accrued Expense	$ 36.9
Target Performance Shares Outstanding, Assumed forfeitures	(647)
Accrued Expense, Assumed forfeitures	(0.5)
Total Target Performance Shares Outstanding	42,473	42,473	40,616	40,616	40,616	50,515
Total Accrued Expense	$ 36.4	$ 30.8	$ 31.7	$ 34.9	$ 27.2	$ 45.8
Performance Cycle 2017 to 2019
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	14,070
Accrued Expense	$ 24.4
Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	13,450
Accrued Expense	$ 8.5
Performance Cycle 2019 to 2021
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	15,600
Accrued Expense	$ 4.0