Third-Party Reinsurance (OneBeacon) (Details) (USD $)					12 Months Ended				12 Months Ended												1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2012 OneBeacon Rating	Dec. 31, 2011 OneBeacon	Dec. 31, 2012 Maximum OneBeacon	Dec. 31, 2012 General Reinsurance Corporation Sirius Group	Dec. 31, 2012 OneBeacon Property Catastrophe Reinsurance Program	Dec. 31, 2011 OneBeacon Property Catastrophe Reinsurance Program	Dec. 31, 2012 OneBeacon Property Catastrophe Reinsurance Program Maximum	Dec. 31, 2012 OneBeacon Property Catastrophe Reinsurance Program Minimum	Dec. 31, 2012 OneBeacon Collector cars and boats property cat treaty [Member]	Dec. 31, 2012 OneBeacon Collector cars and boats property cat treaty [Member] Maximum	Dec. 31, 2012 OneBeacon Collector cars and boats property cat treaty [Member] Amount in excess of minimum [Member]	Dec. 31, 2012 OneBeacon Collector cars and boats property cat treaty [Member] Minimum	Dec. 31, 2012 OneBeacon IMU Treaty	Dec. 31, 2012 OneBeacon IMU Treaty Maximum	Dec. 31, 2012 OneBeacon IMU Treaty Amount in excess of minimum [Member]	Dec. 31, 2012 OneBeacon IMU Treaty Minimum	Jun. 30, 2010 OneBeacon Northeast Homeowners Business	Dec. 31, 2012 OneBeacon Property Per Risk Reinsurance Program	Dec. 31, 2012 OneBeacon Property Per Risk Reinsurance Program Maximum	Dec. 31, 2012 OneBeacon Property Per Risk Reinsurance Program Minimum	Dec. 31, 2012 OneBeacon Casualty Reinsurance Program	Dec. 31, 2012 OneBeacon Casualty Reinsurance Program Maximum	Dec. 31, 2012 OneBeacon Casualty Reinsurance Program Amount in excess of minimum [Member]	Dec. 31, 2012 OneBeacon Casualty Reinsurance Program Minimum	Dec. 31, 2012 OneBeacon Workers compensation [Member]	Dec. 31, 2012 OneBeacon Workers compensation [Member] Maximum	Dec. 31, 2012 OneBeacon Workers compensation [Member] Minimum	Dec. 31, 2012 OneBeacon Casualty Multiple Insured [Member] Amount in excess of minimum [Member]	Dec. 31, 2012 OneBeacon Casualty Multiple Insured [Member] Minimum	Dec. 31, 2012 OneBeacon Professional Liability [Member]	Dec. 31, 2012 OneBeacon Professional Liability [Member] Maximum	Dec. 31, 2012 OneBeacon Professional Liability [Member] Amount in excess of minimum [Member]	Dec. 31, 2012 OneBeacon Professional Liability [Member] Minimum	Dec. 31, 2012 Hurricane Sandy [Member] OneBeacon Property Catastrophe Reinsurance Program
Reinsurance [Line Items]
AM best rating system number of ratings					16
Reinsurance recoverable on paid and unpaid losses	$ 447,000,000	$ 2,537,800,000			$ 110,700,000	$ 2,184,100,000		$ 56,200,000
Percentage of total reinsurance recoverables								17.00%
Percentage of certified losses required to be shared by federal government subject to terrorism risk insurance act					85.00%
Liability to be paid by government and industry subject to terrorism risk insurance act					100,000,000,000		100,000,000,000
Property catastrophe reinsurance program first layer									25,000,000							2,500,000			5,000,000	2,000,000			50,000,000	20,000,000			6,000,000	5,000,000								5,000,000	5,000,000
Property Catastrophe Reinsurance Treaty Second Layer Co-Partipation Percentage													20.00%									20.00%			10.00%
Property catastrophe reinsurance program, second layer									155,000,000						20,000,000	5,000,000								50,000,000			10,000,000	11,000,000
Property catastrophe reinsurance program second layer first retention percentage									55.00%
Property catastrophe reinsurance program first retention slab for second layer											40,000,000	25,000,000
Property catastrophe reinsurance program second layer, second retention percentage									15.00%
Property catastrophe reinsurance program second retention slab for second layer											80,000,000	40,000,000
Property catastrophe reinsurance program second layer third retention percentage									10.00%
Property catastrophe reinsurance program third retention slab for second layer											180,000,000	80,000,000
Property catastrophe reinsurance retention									49,300,000
Property catastrophe reinsurance program third layer										225,000,000
Incurred Losses Ceded, Hurricane													11,900,000				41,000,000																					15,600,000
Property catastrophe reinsurance treaty loss coverage														25,000,000	2,500,000			60,000,000												10,000,000	1,000,000
Property catastrophe treaty aggregate deductible for large losses																	1,500,000
Property Catastrophe Treaty Aggregate Deductible																	5,000,000
Property catastrophe reinsurance treaty half loss layer																			10,000,000	50,000,000
Property catastrophe reinsurance treaty first layer co-partipation percentage													5.00%									10.00%			22.50%									20.00%
Workers compensation treaty coverage percentage																													100.00%
Property catastrophe reinsurance treaty reinstatement premiums													1,400,000				5,400,000																					1,900,000
Property catastrophe treaty per occurance amount																		60,000,000		2,000,000
Estimated individual retention level for commercial plicies, next fiscal year					100,000,000
Ceded premiums written under quota share reinsurance agreement																					25,600,000
Reinsurance program reinsured terrorism losses																							100,000,000	10,000,000
Reinsurance program reinsured losses																										21,000,000		5,000,000							20,000,000		5,000,000
Reinsurance recoverable cover under reinsurance contract																																40,000,000
Amount of insurance losses retained																																	10,000,000
Reinsurance recoverable on paid losses	17,900,000	30,500,000			3,300,000
Reinsurance recoverable on unpaid losses	429,100,000	2,507,300,000	2,344,000,000	2,771,500,000	107,300,000
Reinsurance recoverables on unpaid losses					90.00%
Largest recoverable from reinsurer					$ 10,100,000