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Transactions with Related Persons (Details) - USD ($)
3 Months Ended 12 Months Ended
Sep. 30, 2016
Jun. 30, 2016
Sep. 30, 2015
Jun. 30, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Related Party Transaction [Line Items]              
Assets         $ 6,544,700,000 $ 10,282,600,000  
Sales of common equity securities         $ 1,200,800,000 $ 461,400,000 $ 609,800,000
Treasury Stock Acquired, Average Cost Per Share         $ 802 $ 733 $ 617
OneBeacon              
Related Party Transaction [Line Items]              
Assets         $ 3,589,400,000 $ 3,600,800,000  
Prospector              
Related Party Transaction [Line Items]              
Asset Management Fees           2,100,000 $ 6,500,000
Shares paid or expired(1)           $ 5,500,000  
Percentage of fees in excess of $500,000 revenue chargeable           6.00%  
Amount of revenue over which 6% of fees chargeable           $ 500,000  
Management Fees, Base Revenue           400,000 1,100,000
Incentive fees incurred           100,000 700,000
Franklin Mutual Advisers [Member]              
Related Party Transaction [Line Items]              
Treasury Stock Acquired, Average Cost Per Share $ 820.00 $ 807.00 $ 761.50 $ 650.03      
White Mountains Advisors LLC | Symetra              
Related Party Transaction [Line Items]              
Asset Management Fees           $ 8,000,000 18,400,000
Combination One | Prospector              
Related Party Transaction [Line Items]              
Assets         200,000,000    
Combination Two [Member] | Prospector              
Related Party Transaction [Line Items]              
Assets         200,000,000    
Combination Three [Member] | Prospector              
Related Party Transaction [Line Items]              
Assets         $ 400,000,000    
revenue sharing agreement | Prospector              
Related Party Transaction [Line Items]              
Fees from Revenue Sharing Agreement with Related Party             $ 400,000
First Threshold Limit | White Mountains Advisors LLC | Prospector              
Related Party Transaction [Line Items]              
basis points           1.00%  
Second Threshold Limit | White Mountains Advisors LLC | Prospector              
Related Party Transaction [Line Items]              
basis points           0.50%  
Third Threshold Limit | White Mountains Advisors LLC | Prospector              
Related Party Transaction [Line Items]              
basis points           0.25%  
W T M Incentive Plan | Performance Shares | White Mountains              
Related Party Transaction [Line Items]              
Number of performance shares granted         22,615 29,195 39,590
Shares paid or expired(1)         $ 41,000,000 $ 30,800,000 $ 24,500,000
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         1,106,145 387,495 217,879
Common Stock | Franklin Mutual Advisers [Member]              
Related Party Transaction [Line Items]              
Common shares repurchased and retired during the period 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