XML 102 R91.htm IDEA: XBRL DOCUMENT v3.5.0.2
Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) - USD ($)
$ in Millions
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Share-based compensation arrangement by share-based payment award            
Total Accrued Expense $ 106.3   $ 140.7      
White Mountains | Performance Share | W T M Incentive Plan            
Share-based compensation arrangement by share-based payment award            
Target Performance Shares Outstanding 77,949          
Accrued Expense $ 33.1          
Target Performance Shares Outstanding, Assumed forfeitures (1,948)          
Accrued Expense, Assumed forfeitures (0.9)          
Total Target Performance Shares Outstanding 76,001 111,387 96,358 109,276 108,545 113,295
Total Accrued Expense $ 32.2 $ 71.3 $ 57.7 $ 28.8 $ 25.7 $ 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 16,215          
Accrued Expense $ 2.2          
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 29,478          
Accrued Expense $ 12.7          
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 32,256          
Accrued Expense $ 18.2