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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, 2014
Dec. 31, 2013
Mar. 31, 2013
Dec. 31, 2012
Share-based compensation arrangement by share-based payment award        
Total Accrued Expense $ 124.9 $ 218.3    
White Mountains | Performance Share | W T M Incentive Plan [Member]
        
Share-based compensation arrangement by share-based payment award        
Target Performance Shares Outstanding 130,807      
Accrued Expense 43.4      
Target Performance Shares Outstanding, Assumed forfeitures (3,270)      
Accrued Expense, Assumed forfeitures (1.1)      
Total Target Performance Shares Outstanding 127,537 119,220 118,976 119,357
Total Accrued Expense 42.3 60.2 28.6 29.4
White Mountains | Performance Cycle 2012 To 2014 | Performance Share | W T M Incentive Plan [Member]
        
Share-based compensation arrangement by share-based payment award        
Target Performance Shares Outstanding 37,977      
Accrued Expense 25.0      
White Mountains | Performance Cycle 2014 to 2016 | Performance Share | W T M Incentive Plan [Member]
        
Share-based compensation arrangement by share-based payment award        
Target Performance Shares Outstanding 45,660      
Accrued Expense 2.5      
White Mountains | Performance Cycle 2013 to 2015 | Performance Share | W T M Incentive Plan [Member]
        
Share-based compensation arrangement by share-based payment award        
Target Performance Shares Outstanding 47,170      
Accrued Expense $ 15.9