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	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012
Share-based compensation arrangement by share-based payment award
Total Accrued Expense	$ 157.9		$ 218.3
White Mountains | Performance Share | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	130,807
Accrued Expense	51.8
Target Performance Shares Outstanding, Assumed forfeitures	(3,270)
Accrued Expense, Assumed forfeitures	(1.3)
Total Target Performance Shares Outstanding	127,537	127,537	119,220	118,976	118,976	119,357
Total Accrued Expense	50.5	42.3	60.2	34.9	28.6	29.4
White Mountains | Performance Cycle 2012 To 2014 | Performance Share | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	37,977
Accrued Expense	28.0
White Mountains | Performance Cycle 2014 to 2016 | Performance Share | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	45,660
Accrued Expense	4.6
White Mountains | Performance Cycle 2013 to 2015 | Performance Share | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	47,170
Accrued Expense	$ 19.2