Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Share-based compensation arrangement by share-based payment award
Total Accrued Expense	$ 114.2	$ 141.7
White Mountains | Performance Share | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	114,093
Accrued Expense	$ 73.6
Target Performance Shares Outstanding, Assumed forfeitures	(2,853)
Accrued Expense, Assumed forfeitures	(1.8)
Total Target Performance Shares Outstanding	111,240	96,211	108,398	113,198
Total Accrued Expense	$ 71.8	$ 58.2	$ 25.7	$ 44.4
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	16,215
Accrued Expense	$ 1.0
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	29,428
Accrued Expense	$ 12.8
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2014 to 2016
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	32,157
Accrued Expense	$ 18.7
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2013 to 2015
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	36,293
Accrued Expense	$ 41.1