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Transactions with Related Persons (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Jul. 13, 2017
Sep. 15, 2016
Apr. 19, 2016
Sep. 17, 2015
Jun. 01, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Related Party Transaction [Line Items]                
Value of each common share repurchased and retired during the period (in dollars per share)           $ 869 $ 802 $ 733
Prospector                
Related Party Transaction [Line Items]                
Asset management fees               $ 2.1
Shares paid or expired               5.5
Management fees, base revenue               0.4
Incentive fees incurred               0.1
Franklin Mutual Advisers                
Related Party Transaction [Line Items]                
Value of each common share repurchased and retired during the period (in dollars per share) $ 850.00 $ 820.00 $ 807.00 $ 761.50 $ 650.03      
White Mountains Advisors LLC | Symetra                
Related Party Transaction [Line Items]                
Asset management fees               $ 8.0
W T M Incentive Plan | Performance Shares | White Mountains                
Related Party Transaction [Line Items]                
Number of performance shares granted           17,710 22,615 29,195
Shares paid or expired           $ 21.9 $ 41.0 $ 30.8
W T M Incentive Plan | Performance Shares | White Mountains | Prospector                
Related Party Transaction [Line Items]                
Number of performance shares granted               7,000
Common Stock                
Related Party Transaction [Line Items]                
Common shares repurchased and retired during the period           832,725 1,106,145 387,495
Common Stock | Franklin Mutual Advisers                
Related Party Transaction [Line Items]                
Common shares repurchased and retired during the period 235,000 305,000 325,000 26,300 19,688      
2014-2016 Cycle | W T M Incentive Plan | Performance Shares | White Mountains | Prospector                
Related Party Transaction [Line Items]                
Number of performance shares granted               6,250