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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

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