Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Share-based compensation arrangement by share-based payment award
Total Accrued Expense	$ 124.2		$ 140.7
White Mountains | Performance Share | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	81,577
Accrued Expense	$ 38.4
Target Performance Shares Outstanding, Assumed forfeitures	(2,040)
Accrued Expense, Assumed forfeitures	(1.0)
Total Target Performance Shares Outstanding	79,537	73,297	93,654	93,654	106,572	111,257
Total Accrued Expense	$ 37.4	$ 32.5	$ 57.7	$ 52.5	$ 28.8	$ 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	19,615
Accrued Expense	$ 3.4
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	31,795
Accrued Expense	$ 15.1
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	30,167
Accrued Expense	$ 19.9