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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 |

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