Transactions with Related Persons (Details) (USD $)	12 Months Ended						12 Months Ended										12 Months Ended								12 Months Ended	1 Months Ended	12 Months Ended								0 Months Ended			0 Months Ended	1 Months Ended
	Dec. 31, 2012 Y	Dec. 31, 2011	Jan. 07, 2011	Dec. 31, 2012 OneBeacon	Dec. 31, 2011 OneBeacon	Jan. 07, 2011 Franklin Mutual Advisers	Dec. 31, 2012 Symetra	Dec. 31, 2011 Symetra	Dec. 31, 2010 Symetra	Dec. 31, 2012 Oakum Bay Capital	Dec. 31, 2012 Limited Liability Company	Dec. 31, 2011 Limited Liability Company	Dec. 31, 2010 Limited Liability Company	Dec. 31, 2012 Limited Liability Company OneBeacon	Dec. 31, 2012 White Mountains Advisors LLC Limited Liability Company	Jul. 16, 2012 Oakum Bay Capital	Dec. 31, 2012 Oakum Bay Capital Director	Dec. 31, 2012 Oakum Bay Capital Brother of Director	Dec. 31, 2012 Trimarc Capital Fund	Dec. 31, 2010 Trimarc Capital Fund	Dec. 31, 2006 White Mountains Specialty Underwriting Inc	Dec. 31, 2006 One Beacon Professional Insurance	Dec. 31, 2009 Dowling and Partners Connecticut Fund III Lp Director	Dec. 31, 2012 Dowling and Partners Connecticut Fund III Lp Director	Dec. 31, 2012 Range Of Performance Cycle From 2013 to 2015 [Member] Limited Liability Company	Feb. 28, 2013 Performance Cycle From 2010 to 2012 Limited Liability Company	Dec. 31, 2012 Performance Cycle From 2010 to 2012 Limited Liability Company	Dec. 31, 2010 Performance Cycle From 2010 to 2012 Limited Liability Company	Dec. 31, 2012 Performance Cycle From 2011 to 2013 Limited Liability Company	Dec. 31, 2012 Performance Cycle From 2012 to 2014 Limited Liability Company	Dec. 31, 2012 Maximum Limited Liability Company	Dec. 31, 2012 Minimum Limited Liability Company	Dec. 08, 2012 Treasury Stock	Jul. 12, 2012 Treasury Stock	Dec. 18, 2012 Treasury Stock Beneficial Owner	Jul. 12, 2012 Treasury Stock Beneficial Owner	Dec. 31, 2012 Hedge funds Oakum Bay Capital	Jul. 16, 2012 Preferred Equity Securities Oakum Bay Capital	Dec. 31, 2012 Preferred Equity Securities Oakum Bay Capital
Related Party Transaction [Line Items]
Advisor fees charged on first $200 million of assets under management (as a percent)															100.00%
First threshold limit of assets under management															$ 200,000,000
Advisors fees charged on second $200 million of assets under management (as a percent)															50.00%
Second threshold limit of assets under management															200,000,000
Advisors fees charged on over $400 million of assets under management (as a percent)															25.00%
Third threshold limit of assets under management															400,000,000
Assets under management							27,400,000,000	26,300,000,000						533,600,000	465,900,000
ERISA Assets														205,900,000
Asset management total fees earned							15,900,000	15,200,000	13,800,000		6,400,000	6,000,000	6,600,000
Shares purchased																																			285,000	50,000
Share price																																	$ 519.06	$ 528.45
Percentage of target number granted actually received by prospector																											49.00%
Number of performance shares granted																									7,000			9,200	8,500	7,750
Target number of performance shares granted (as a percent)																															200.00%	0.00%
Payment made with respect to the performance shares granted to the related party																										2,500,000
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											500,000	200,000	700,000
Invested in limited partnership investment interests											117,200,000			12,900,000
Management fees incurred														1,100,000
Incentive fees incurred														1,300,000
Investments Owned by Related Party																																					66,000,000
Total investments	7,278,100,000	8,268,000,000		2,291,500,000	2,707,600,000														9,900,000
Management fees																			56,811
Incentive fees																			88,090
Mangagment and incentive rees received, percent										40.00%
Purchases																			6,000,000	4,000,000																		2,000,000
Ownerhsip percentage																7.50%
Annual dividend rate																																							5.00%
Annual dividend rate after year four																																							10.00%
Years Before Dividend Increase	4
Years Until Investments Madatorily Redeemable	6
Related party ownerhship percentage																	27.50%	45.00%
Number of shares repurchased						140,000
Purchase price per share of shares repuchased (in dollars per share)						$ 342.50
Related Party Transaction Excess of Market Price Over Purchase Price Per Share of Shares Agreed to be Repurchased			$ 2.50
Interst in Dowling & Partners Connecticut Fund III, LP (as a percent)																								33.00%
Borrowings from Fund III																					7,000,000	8,000,000
Tax credits generated by related party																							$ 500,000