Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) (USD $) In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Share-based compensation arrangement by share-based payment award
Total Accrued Expense	$ 100.5	$ 184.6
White Mountains | Performance Share | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	119,380
Accrued Expense	28.3
Target Performance Shares Outstanding, Assumed forfeitures	(2,984)
Accrued Expense, Assumed forfeitures	(0.7)
Total Target Performance Shares Outstanding	116,396	123,549	127,537	119,220
Total Accrued Expense	27.6	48.1	42.3	60.2
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	30,640
Accrued Expense	1.8
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	41,937
Accrued Expense	5.6
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	46,803
Accrued Expense	$ 20.9