Reserves for Unpaid Losses and Loss Adjustment Expenses (Details) (USD $)	12 Months Ended										12 Months Ended		1 Months Ended			12 Months Ended			1 Months Ended		12 Months Ended			12 Months Ended											1 Months Ended				12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jun. 02, 2001 One Beacon	Apr. 16, 2004 Scandinavian Re	Dec. 22, 2006 Stockbridge	Dec. 31, 2011 AutoOne	Dec. 31, 2010 AutoOne	Dec. 31, 2009 AutoOne	Dec. 31, 2008 AutoOne	Dec. 31, 2010 Old Lyme	Dec. 31, 2010 Central National	Dec. 31, 2010 Other lines of business	Dec. 31, 2010 Multiple peril liability	Dec. 31, 2009 Multiple peril liability	Dec. 31, 2010 Multiple peril liability	Dec. 31, 2009 Multiple peril liability	Dec. 31, 2008 Multiple peril liability	Dec. 31, 2010 General liability	Dec. 31, 2009 General liability	Dec. 31, 2010 General liability	Dec. 31, 2009 General liability	Dec. 31, 2008 General liability	Dec. 31, 2011 Professional liability business	Dec. 31, 2010 Professional liability business	Dec. 31, 2009 Professional liability business	Dec. 31, 2011 Commercial lines underwriting unit	Dec. 31, 2010 Commercial lines underwriting unit	Dec. 31, 2009 Commercial lines underwriting unit	Dec. 31, 2011 OneBeacon	Dec. 31, 2010 OneBeacon	Dec. 31, 2009 OneBeacon	Dec. 31, 2011 OneBeacon Specialty insurance operations	Dec. 31, 2011 OneBeacon Other lines of business	Dec. 31, 2009 OneBeacon Multiple peril liability	Dec. 31, 2008 OneBeacon Multiple peril liability	Dec. 31, 2009 OneBeacon General liability	Dec. 31, 2008 OneBeacon General liability	Dec. 31, 2011 Sirius group	Dec. 31, 2010 Sirius group	Dec. 31, 2009 Sirius group
Loss and Loss Adjustment Expense Reserve
Gross beginning balance	$ 5,736,800,000	$ 6,379,200,000	$ 7,029,400,000																																				$ 2,441,300,000
Less beginning reinsurance recoverable on unpaid losses	(2,344,000,000)	(2,771,500,000)	(3,049,000,000)
Net loss and LAE reserves	3,392,800,000	3,607,700,000	3,980,400,000				0	77,300,000	95,800,000	102,000,000
Loss and LAE reserves acquired											21,000,000	17,600,000
Loss and LAE reserves sold - OneBeacon Personal Lines		(231,000,000)
Loss and LAE incurred relating to current year losses	1,256,700,000	1,503,400,000	1,571,200,000
Net favorable loss reserve development	(49,800,000)	(114,200,000)	(147,100,000)																											2,900,000	57,100,000	116,700,000	29,600,000	(26,700,000)					46,900,000	57,100,000	30,400,000
Total incurred losses and LAE	1,206,900,000	1,389,200,000	1,424,100,000													65,800,000	36,800,000				33,100,000	15,600,000																	626,000,000	531,000,000	418,800,000
Accretion of fair value adjustment to net loss and LAE reserves	8,300,000	8,500,000	12,200,000
Foreign currency translation adjustment to net loss and LAE reserves	100,000	4,700,000	23,500,000
Loss and LAE paid relating to current year losses	(404,000,000)	(544,900,000)	(591,600,000)
Loss and LAE paid relating to prior year losses	(952,800,000)	(840,500,000)	(1,234,700,000)
Total loss and LAE payments	(1,356,800,000)	(1,385,400,000)	(1,826,300,000)
Net ending balance	3,195,000,000	3,392,800,000	3,607,700,000				0	77,300,000	95,800,000	102,000,000
Plus ending reinsurance recoverable on unpaid losses	(2,507,300,000)	(2,344,000,000)	(2,771,500,000)																																				(339,700,000)
Gross ending balance	5,702,300,000	5,736,800,000	6,379,200,000																																				2,343,700,000	2,441,300,000
Estimated IBNR														86,200,000	170,500,000	86,200,000	170,500,000		61,700,000	107,100,000	61,700,000	107,100,000													95,400,000	173,900,000	61,600,000	93,900,000
IBNR as a percentage of case reserves														57.00%	93.00%				164.00%	265.00%															73.00%	85.00%	214.00%	275.00%
Case reserves															184,100,000		184,100,000	204,900,000		40,400,000		40,400,000	34,200,000
Case incurred loss and ALAE during the period	1,206,900,000	1,389,200,000	1,424,100,000													65,800,000	36,800,000				33,100,000	15,600,000																	626,000,000	531,000,000	418,800,000
Increase (decrease) in reserves													6,400,000			(18,400,000)	(42,600,000)				(12,300,000)	(14,100,000)		(11,500,000)	(19,300,000)	(60,000,000)	(14,000,000)	(7,500,000)	7,500,000	(4,100,000)
Net favorable loss reserve development due to property lines																																							41,200,000
Net favorable loss reserve development due to 2010 Chilean earthquake																																							13,100,000
Net unfavorable loss reserve development due to asbestos and environmental reserves additions																																							12,300,000
Deferred gains from a retrocessional reinsurance contract																																								16,300,000
Losses incurred related to A&E exposures																																									17,700,000
Losses ceded under a retrocessional reinsurance contract																																									20,000,000
Retroceded premiums																																									10,000,000
Interest charges on funds held																																									7,200,000
Pre-tax un-accreted adjustment	$ 12,700,000
Purchase accounting valuation of loss and LAE reserves, cash flow discount rate				11.00%	12.00%	2.00%