Third Party Reinsurance (Sirius Group) (Details)	12 Months Ended				12 Months Ended									1 Months Ended	12 Months Ended	23 Months Ended	1 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended																			12 Months Ended
	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2012 Sirius Group USD ($)	Dec. 31, 2011 Sirius Group USD ($)	Dec. 31, 2010 Sirius Group USD ($)	Dec. 31, 2012 Sirius Group USD ($)	Dec. 31, 2011 Sirius Group USD ($)	Dec. 31, 2010 Sirius Group USD ($)	Dec. 31, 2012 OneBeacon USD ($) Rating	Dec. 31, 2011 OneBeacon USD ($)	Dec. 31, 2010 OneBeacon USD ($)	Apr. 30, 2012 Property Catastrophe Reinsurance Program Sirius Group USD ($)	Dec. 31, 2012 Property Catastrophe Reinsurance Program Sirius Group USD ($)	Dec. 31, 2012 Property Catastrophe Reinsurance Program Sirius Group USD ($)	Apr. 30, 2010 New Zealand Earthquake Loss Cover Sirius Group USD ($)	Dec. 31, 2012 Nonproportional Aviation Portfolio Reinsurance Sirius Group USD ($)	Dec. 31, 2012 Minimum Sirius Group USD ($)	Apr. 30, 2011 Minimum Property Catastrophe Reinsurance Program Sirius Group USD ($)	Dec. 31, 2012 Minimum Facultative and Proportional Aviation Portfolio Reinsuranc Sirius Group USD ($)	Dec. 31, 2012 Minimum Nonproportional Aviation Portfolio Reinsurance Sirius Group USD ($)	Dec. 31, 2012 Minimum Personal Accident and Life Protection Sirius Group USD ($)	Dec. 31, 2012 Minimum Personal Accident and Life Protection Sirius Group EUR (€)	Dec. 31, 2012 Amount in excess of minimum [Member] Sirius Group USD ($)	Dec. 31, 2012 Amount in excess of minimum [Member] Property Catastrophe Reinsurance Program Sirius Group USD ($)	Dec. 31, 2012 Amount in excess of minimum [Member] Personal Accident and Life Protection Sirius Group USD ($)	Dec. 31, 2012 Amount in excess of minimum [Member] Personal Accident and Life Protection Sirius Group EUR (€)	Dec. 31, 2012 Maximum Facultative and Proportional Aviation Portfolio Reinsuranc Sirius Group USD ($)	Dec. 31, 2012 Maximum Nonproportional Aviation Portfolio Reinsurance Sirius Group USD ($)	Dec. 31, 2012 Tokio Marine and Nichido Fire	Dec. 31, 2012 General Reinsurance Corporation Sirius Group USD ($)	Dec. 31, 2012 Swiss Re Group Sirius Group USD ($)	Dec. 31, 2012 Olympus Sirius Group USD ($)		Dec. 31, 2012 Lloyds of London Sirius Group USD ($)		Dec. 31, 2012 Michigan Catastorphic Claims Association Sirius Group USD ($)		Dec. 31, 2012 Non US and Non Japan Sirius Group USD ($)	Dec. 31, 2012 New Madrid Sirius Group USD ($)	Dec. 31, 2012 European Windstorms and Flood Sirius Group USD ($)	Dec. 31, 2012 London Branch Minimum Sirius Group USD ($)	Dec. 31, 2012 London Branch Amount in excess of minimum [Member] Sirius Group USD ($)	Dec. 31, 2012 Hurricane Sandy [Member] Sirius Group USD ($)
Reinsurance [Line Items]
AM best rating system number of ratings											16
Loss reinsurance protection, amount																			$ 5,000,000						$ 15,000,000
Additional Reinsurance Protection Purchased for 2013 for the London Facultative branch																			20,000,000
Additional Reinsurance Protection for the London branch for facultative and direct U.S.-catastrophe exposed business (excluding Florida risks)																									5,000,000
Insurance amount In excess of retention																																											300,000
Insurance recoveries															2,000,000																														5,000,000
Reinsurance recoverable on paid and unpaid losses	447,000,000	2,537,800,000						336,300,000	353,700,000		110,700,000	2,184,100,000																				56,200,000	36,400,000	29,600,000	[1]	19,400,000	[2]	14,700,000	[3]
Reinsurance recoverable cover under reinsurance contract														17,000,000			65,000,000						7,000,000	5,000,000		40,000,000	13,000,000	10,000,000
Reinsurance program reinsured losses																					150,000,000								2,250,000,000
Amount of insurance losses retained																				35,000,000
Reinsurance program cover in first event industry loss warranties																																								10,000,000		5,000,000,000
Industry Loss Trigger for Reinsurance Program Cover in Earthquake Loss Warranties																																								7,500,000,000
Reinsurance program cover in second event industry loss warranties																																								5,000,000
Reinsurance program cover in earthquake loss warranties																																									37,500,000
Reinsurance program cover in first and second event industry loss warranties								20,000,000,000
Reinsurance program market loss event																																										5,000,000
Loss and loss adjustment expenses	1,193,900,000	1,174,300,000	1,216,600,000		543,900,000	626,000,000	531,000,000	543,900,000	626,000,000	531,000,000	650,000,000	548,300,000	685,600,000			47,000,000
Percentage of quota share reinsurance agreement																		15.00%
Reinsurance coverage																																										40,000,000
Percentage of trade credit business ceded under quota share retrocession					20.00%			20.00%
Percentage of bond business ceded under renewed quota share retrocession					50.00%			50.00%
Percentage of Property Catastrophe Reinsurance Treaty Reinstatement Premiums																															100.00%
Reinsurance recoverable on paid losses	17,900,000	30,500,000						14,600,000			3,300,000
Reinsurance recoverable on unpaid losses	429,100,000	2,507,300,000	2,344,000,000	2,771,500,000				321,800,000			107,300,000
Percentage of total reinsurance recoverables																																17.00%	11.00%	9.00%	[1]	6.00%	[2]	4.00%	[3]
Percentage collateralized																																0.00%	2.00%	100.00%	[1]	8.00%	[2]	0.00%	[3]
Seller note from acquisition	25,000,000
Aviation reinsurance program first event industry loss warranties required industry losses																		33,000,000
Aviation reinsurance program cover in second event industry loss warranties																		5,000,000
Aviation reinsurance program cover in first event industry loss warranties																						400,000,000								1,000,000,000
Reinsurance Coverage for Marine Yachat Portfolio																																												$ 9,800,000

[1] Non-U.S. insurance entity. The balance is fully collateralized through funds held, letters of credit or trust agreements.

[2] Represents the total of reinsurance recoverables due to Sirius Group from all Lloyds Syndicates.

[3] Michigan Catastrophic Claims Association (”MCCA”) is a non-profit unincorporated association., established by the State of Michigan with the power to issue and collect assessments, to which every insurance company that sells automobile coverage in Michigan is required to be a member. A.M. Best does not rate MCCA. Sirius Group acquired its recoverable from MCCA in the acquisition of Stockbridge Insurance Company. As part of the acquisition, Sirius Group obtained $25.0 of reinsurance protection from the seller (currently rated A+ by A.M. Best) for unfavorable loss reserve development, including uncollectible reinsurance.