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
	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