Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Share-based compensation arrangement by share-based payment award
Total Accrued Expense	$ 83.2	$ 140.0
White Mountains | Performance Share | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	57,670
Accrued Expense	$ 24.3
Target Performance Shares Outstanding, Assumed forfeitures	(865)
Accrued Expense, Assumed forfeitures	(0.3)
Total Target Performance Shares Outstanding	56,805	80,353	108,683	93,654
Total Accrued Expense	$ 24.0	$ 42.4	$ 71.3	$ 57.7
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	16,460
Accrued Expense	$ 1.2
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	19,415
Accrued Expense	$ 7.3
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	21,795
Accrued Expense	$ 15.8