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Transactions with Related Persons (Details) - USD ($) |  | 6 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Jul. 16, 2012 | Jun. 30, 2015 | Sep. 30, 2015 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Assets |  |  |  | $ 10,284,500,000 | $ 10,455,700,000 |  |
Purchases |  |  |  | 2,377,100,000 | (2,868,500,000) |  |
Sales of common equity securities |  |  |  | $ 461,400,000 | $ 609,800,000 | $ 435,000,000 |
Treasury Stock Acquired, Average Cost Per Share |  |  |  | $ 733 | $ 617 | $ 564 |
OneBeacon |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Assets |  |  |  | $ 3,602,700,000 | $ 3,559,800,000 |  |
Prospector |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Assets under management |  |  |  |  | 82,400,000 |  |
Asset Management Fees |  |  |  | 2,100,000 | 6,500,000 | $ 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 | 1,300,000 |
Incentive fees incurred |  |  |  | 100,000 | 700,000 | 3,200,000 |
Franklin Mutual Advisers [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Treasury Stock Acquired, Average Cost Per Share |  | $ 650.03 | $ 761.50 |  |  |  |
OneBeacon | Prospector |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Assets under management |  |  |  |  | 15,800,000 |  |
White Mountains Advisors LLC | Symetra |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Assets under management |  |  |  | 22,100,000,000 | 30,900,000,000 |  |
Asset Management Fees |  |  |  | 8,000,000 | 18,400,000 | 16,500,000 |
Oakum Bay Capital |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Ownerhsip percentage | 7.50% |  |  |  |  |  |
Sales of common equity securities |  |  |  |  |  | $ 500,000 |
Number of years to receive portion of revenue |  |  |  |  |  | 10 years |
Oakum Bay Capital | Director |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Related party ownerhship percentage | 67.50% |  |  |  |  |  |
Combination One [Member] | Prospector |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Assets |  |  |  | 200,000,000 | 195,700,000 |  |
Combination Two [Member] | Prospector |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Assets |  |  |  | 200,000,000 | 431,200,000 |  |
Combination Three [Member] | Prospector |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Assets |  |  |  | 400,000,000 |  |  |
Preferred Equity Securities | Oakum Bay Capital |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Purchases | $ 2,000,000 |  |  |  |  |  |
revenue sharing agreement | Prospector |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Fees from Revenue Sharing Agreement with Related Party |  |  |  |  | $ 400,000 | $ 900,000 |
Trimarc Capital Fund | Hedge funds | Oakum Bay Capital |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Trading Securities, Other |  |  |  | $ 8,900,000 |  |  | $ 9,500,000
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 |  |  |  | 29,828 | 41,580 | 43,660 |
Shares paid or expired(1) |  |  |  | $ (30,800,000) | $ (24,500,000) | $ (10,100,000) |
W T M Incentive Plan | Performance Shares | White Mountains | Prospector |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Number of performance shares granted |  |  |  | 7,000 |  |  |
ERISA assets [Member] | Combination Two [Member] | Prospector |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Assets |  |  |  |  | $ 202,900,000 |  |
Total |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Common shares repurchased and retired during the period |  |  |  | 387,495 | 217,879 | 141,535 |
Total | Franklin Mutual Advisers [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Common shares repurchased and retired during the period |  | 19,688 | 26,300 |  |  |  |
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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