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Transactions with Related Persons (Details) (USD $) | 12 Months Ended | 3 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 9 Months Ended |  | 12 Months Ended |  | 3 Months Ended | 0 Months Ended | 12 Months Ended |
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Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Mar. 31, 2013 Beneficial Owner | Mar. 15, 2013 Beneficial Owner | Dec. 31, 2013 Prospector | Dec. 31, 2012 Prospector | Dec. 31, 2011 Prospector | Dec. 31, 2013 OneBeacon Prospector | Dec. 31, 2013 White Mountains Advisors LLC Symetra | Dec. 31, 2012 White Mountains Advisors LLC Symetra | Dec. 31, 2011 White Mountains Advisors LLC Symetra | Dec. 31, 2013 White Mountains Advisors LLC Prospector | Sep. 30, 2013 Oakum Bay Capital | Jul. 16, 2012 Oakum Bay Capital | Dec. 31, 2012 Oakum Bay Capital Director | Dec. 31, 2013 Performance Cycle From 2012 to 2014 Prospector | Dec. 31, 2013 Performance Cycle From 2013 to 2015 Prospector | Dec. 31, 2013 First Threshold Limit White Mountains Advisors LLC Prospector | Dec. 31, 2013 Second Threshold Limit White Mountains Advisors LLC Prospector | Dec. 31, 2013 Third Threshold Limit White Mountains Advisors LLC Prospector | Dec. 31, 2013 Maximum Prospector | Dec. 31, 2013 Minimum Prospector | Mar. 31, 2014 Range Of Performance Cycle From 2014-2016 Prospector | Mar. 31, 2014 Performance Cycle From 2011 to 2013 Prospector | Jul. 16, 2012 Preferred Equity Securities Oakum Bay Capital | Dec. 31, 2013 revenue sharing agreement Prospector | Dec. 31, 2012 revenue sharing agreement Prospector | Dec. 31, 2011 revenue sharing agreement Prospector | Dec. 31, 2013 Trimarc Capital Fund Oakum Bay Capital | Dec. 31, 2012 Trimarc Capital Fund Oakum Bay Capital | Dec. 31, 2013 Trimarc Capital Fund Hedge funds Oakum Bay Capital
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
basis points |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% | 50.00% | 25.00% |  |  |  |  |  |  |  |  |  |  |
Assets under management |  |  |  |  |  |  |  |  | $ 556,800,000 | $ 27,800,000,000 | $ 27,400,000,000 |  | $ 461,000,000 |  |  |  |  |  | $ 200,000,000 | $ 200,000,000 | $ 400,000,000 |  |  |  |  |  |  |  |  |  |  |
ERISA Assets |  |  |  |  |  |  |  |  | 215,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Asset Management Fees |  |  |  |  |  | 6,500,000 | 6,400,000 | 6,000,000 |  | 16,500,000 | 15,900,000 | 15,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of target number granted actually received by prospector |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 131.50% |  |  |  |  |  |  |
Number of performance shares granted |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7,750 | 7,000 |  |  |  |  |  | 6,250 | 8,500 |  |  |  |  |  |  |
Target number of performance shares granted (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 200.00% | 0.00% |  |  |  |  |  |  |  |  |
Per share price, class of warrant (in dollars per share) |  |  |  |  | $ 563.57 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment made with respect to the performance shares granted to the related party |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6,500,000 |  |  |  |  |  |  |
Performance shares granted at the beginning of each performance cycle |  |  |  |  |  |  | 4,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of fees in excess of $500,000 revenue chargeable |  |  |  |  |  | 6.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of revenue over which 6% of fees chargeable |  |  |  |  |  |  | 500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fees from Revenue Sharing Agreement with Related Party |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 900,000 | 500,000 | 200,000 |  |  |
Invested in limited partnership investment interests |  |  |  |  |  | 108,400,000 |  |  | 14,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management Fees, Base Revenue |  |  |  |  |  | 1,300,000 | 1,100,000 | 1,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incentive fees incurred |  |  |  |  |  | 3,200,000 | 1,300,000 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100,000 | 100,000 |
Purchases |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,000,000 |  |  |  |  |  |
Ownerhsip percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party ownerhship percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 67.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sales of common equity securities | 521,100,000 | 192,400,000 | 237,800,000 |  |  |  |  |  |  |  |  |  |  | 500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of years to receive portion of revenue |  |  |  |  |  |  |  |  |  |  |  |  |  | 10 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares purchased |  |  |  | 140,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Trading Securities, Other |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 8,600,000

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