Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Share-based compensation arrangement by share-based payment award
Total Accrued Expense	$ 121.0		$ 108.1
White Mountains | Performance Share | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	98,678
Accrued Expense	$ 54.0
Target Performance Shares Outstanding, Assumed forfeitures	(2,467)
Accrued Expense, Assumed forfeitures	(1.5)
Total Target Performance Shares Outstanding	96,211	109,129	113,198	116,259	116,259	108,605
Total Accrued Expense	$ 52.5	$ 28.8	$ 44.4	$ 52.4	$ 46.3	$ 54.9
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	29,828
Accrued Expense	$ 6.3
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	32,557
Accrued Expense	$ 12.3
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	$ 35.4