Third-Party Reinsurance (Details 2)	12 Months Ended					12 Months Ended		12 Months Ended					127 Months Ended			12 Months Ended			1 Months Ended	12 Months Ended						1 Months Ended	12 Months Ended	127 Months Ended	12 Months Ended		12 Months Ended			12 Months Ended			12 Months Ended					1 Months Ended		12 Months Ended	1 Months Ended	12 Months Ended
	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2007 Y	Dec. 31, 2008 USD ($)	Dec. 31, 2011 Healthcare professional liability USD ($)	Dec. 31, 2011 NICO Cover contract	Dec. 31, 2010 Imagine Re Sirius America USD ($)	Dec. 31, 2009 Imagine Re Sirius America USD ($)	Dec. 31, 2011 OneBeacon USD ($)	Dec. 31, 2010 OneBeacon USD ($)	Dec. 31, 2009 OneBeacon USD ($)	Dec. 31, 2011 OneBeacon USD ($)	Dec. 31, 2001 OneBeacon rating contract	Dec. 31, 2011 OneBeacon Maximum USD ($)	Dec. 31, 2011 OneBeacon Property catastrophe reinsurance program USD ($)	Dec. 31, 2011 OneBeacon Property catastrophe reinsurance program Minimum USD ($)	Dec. 31, 2011 OneBeacon Property catastrophe reinsurance program Maximum USD ($)	Jun. 30, 2010 OneBeacon Northeast homeowners business USD ($)	Dec. 31, 2009 OneBeacon Northeast homeowners business USD ($)	Dec. 31, 2011 OneBeacon Property-per-risk reinsurance program USD ($)	Dec. 31, 2011 OneBeacon Property-per-risk reinsurance program Minimum USD ($)	Dec. 31, 2011 OneBeacon Property-per-risk reinsurance program Maximum USD ($)	Dec. 31, 2011 OneBeacon Casualty reinsurance program Minimum USD ($)	Dec. 31, 2011 OneBeacon Casualty reinsurance program Maximum USD ($)	Jun. 30, 2001 OneBeacon NICO Cover USD ($)	Dec. 31, 2011 OneBeacon NICO Cover USD ($)	Dec. 31, 2011 OneBeacon NICO Cover USD ($)	Dec. 31, 2011 OneBeacon NICO Cover Maximum USD ($)	Dec. 31, 2011 OneBeacon GRC Cover USD ($)	Dec. 31, 2011 OneBeacon GRC Cover Maximum USD ($)	Dec. 31, 2011 OneBeacon National Indemnity Company and General Reinsurance Corporation USD ($)	Dec. 31, 2011 OneBeacon Hanover Insurance Group Ltd. USD ($)	Dec. 31, 2011 OneBeacon Tokio Marine and Nichido Fire USD ($)	Dec. 31, 2011 OneBeacon Tower Group Inc. USD ($)	Dec. 31, 2011 OneBeacon Munich Reinsurance America USD ($)	Dec. 31, 2011 OneBeacon General Reinsurance Corporation GRC Cover USD ($)	Dec. 31, 2010 OneBeacon General Reinsurance Corporation GRC Cover USD ($)	Dec. 31, 2011 Sirius group USD ($)	Dec. 31, 2010 Sirius group USD ($)	Dec. 31, 2009 Sirius group USD ($)	Apr. 30, 2011 Sirius group Catastrophe excess of loss cover USD ($) Y	Apr. 30, 2010 Sirius group New Zealand earthquake losses cover USD ($)	Dec. 31, 2011 Sirius group New Zealand earthquake losses cover USD ($)	Apr. 30, 2009 Sirius group Chile earthquake losses cover USD ($)	Dec. 31, 2010 Sirius group Chile earthquake losses cover USD ($)	Dec. 31, 2012 Sirius group Facultative and proportional aviation portfolio reinsurance Minimum USD ($)	Dec. 31, 2012 Sirius group Facultative and proportional aviation portfolio reinsurance Maximum USD ($)	Dec. 31, 2011 Sirius group Non-proportional aviation portfolio reinsurance USD ($)	Dec. 31, 2011 Sirius group Non-proportional aviation portfolio reinsurance Minimum USD ($)	Dec. 31, 2011 Sirius group Non-proportional aviation portfolio reinsurance Maximum USD ($)	Dec. 31, 2011 Sirius group Personal accident and life USD ($)	Dec. 31, 2011 Sirius group Personal accident and life EUR (€)	Dec. 31, 2011 Sirius group Olympus USD ($)	Dec. 31, 2011 Sirius group Lloyds of London USD ($)	Dec. 31, 2011 Sirius group General Reinsurance Corporation USD ($)	Dec. 31, 2011 Sirius group Swiss Re Group USD ($)	Dec. 31, 2011 Sirius group Michigan Catastrophic Claims Association USD ($)
Third-Party Reinsurance
Period for which the U.S. government extended the Terrorism Act (in years)				7
Percentage of certified losses required to be shared by federal government under Terrorism Risk Insurance Act										85.00%
Aggregate liability to be paid by the government and industry without further action by Congress															$ 100,000,000,000
Estimated individual retention level for commercial policies subject to the Terrorism Act										100,000,000			100,000,000
Amount of loss on a single catastrophe retained, first layer																50,000,000
Amount of loss on a single catastrophe reinsured, second layer																175,000,000
First retention percentage of loss on a single catastrophe reinsured, second layer																26.00%
Amount of loss on a single catastrophe reinsured, first retention slab for second layer																	50,000,000	100,000,000
Second retention percentage of loss on a single catastrophe reinsured, second layer																10.00%
Amount of loss on a single catastrophe reinsured, second retention slab for second layer																	100,000,000	175,000,000
Amount of loss on a single catastrophe retained, third layer																225,000,000
The percentage ceded under quota share reinsurance agreement																				30.00%														100.00%															15.00%
Premiums ceded under quota share agreement																			25,600,000	59,900,000
Reinsurance program, reinsured losses																								5,000,000	21,000,000																						150,000,000	2,250,000,000
Amount above which individual risk facultative reinsurance may be purchased																					100,000,000
Reinsurance protection on an individual risk basis for terrorism losses																						10,000,000	100,000,000
Number of reinsurance contracts the seller caused OneBeacon to purchase immediately prior to the entity's acquisition of OneBeacon							1							2
Number of ratings in Standard & Poors best ratings														21
Number of ratings in A.M. Best ratings														15
Period for which reinsurance cover was renewed (in years)																																										1
Cover under reinsurance contract						20,000,000												40,000,000								2,500,000,000			2,500,000,000		570,000,000											61,000,000	65,000,000		65,000,000							13,000,000	10,000,000
Amount of retention of losses by the entity																		10,000,000																								35,000,000	45,000,000	45,000,000	35,000,000							7,000,000	5,000,000
Amount of earthquake losses at which reinsurance will be fully recovered																																										110,000,000
Ceded estimated incurred losses																											2,300,000,000	2,300,000,000		562,000,000
Reinsurance receivables paid by the reinsurer													1,400,000,000																								73,500,000	61,300,000
Percentage of utilized coverage that relates to uncollectible Third Party Recoverables																												8.00%
Utilized coverage from that relates to uncollectible Third-Party Recoverables																											184,000,000
Utilized coverage																																														65,000,000
Utilized asbestos and environmental coverage																												2,300,000,000
Reinsurance recoverable on paid losses	30,500,000	63,100,000								16,500,000			16,500,000																										13,900,000
Reinsurance recoverable on unpaid losses	2,507,300,000	2,344,000,000	2,771,500,000		3,049,000,000					2,330,800,000			2,330,800,000																										339,700,000
Purchase accounting adjustments										163,300,000			163,300,000
Reinsurance recoverables	2,537,800,000	2,407,100,000								2,184,100,000	1,937,700,000		2,184,100,000																			1,540,600,000	90,900,000	57,100,000	35,100,000	26,400,000			353,700,000	469,400,000														35,500,000	27,200,000	42,700,000	40,200,000	14,600,000
Percentage of total paid and unpaid reinsurance recoverables																																66.00%	4.00%	2.00%	1.00%	1.00%																		10.00%	8.00%	12.00%	11.00%	4.00%
Percentage of reinsurance recoverable that is collateralized																																																						100.00%	8.00%	1.00%	20.00%
Third-Party Recoverables in which reinsurer would pay if they are unable to collect from third-party reinsurers																											198,300,000	198,300,000
Reinsurance recoverable from various third-party reinsurers, guaranteed by the third-party reinsurer under reinsurance agreement																																		30,500,000
Protection in first event industry loss warranties (ILW)																																																		400,000,000	900,000,000
Industry event level for first event industry loss warranties (ILW)																																																	30,000,000
Protection in second event industry loss warranties (ILW)																																																	6,000,000
Industry event level for second event industry loss warranties (ILW)																																																		300,000,000	600,000,000
Percentage of trade credit business under a quota share retrocession																																							19.00%
Percentage of trade credit business under a renewed quota share retrocession																																							20.00%
Percentage of bond business under a renewed quota share retrocession																																							50.00%
Amount of unamortized deferred gains									16,300,000
Amount of deferred gains recognized								16,300,000	4,600,000																															(16,300,000)
Case incurred loss and ALAE during the period	$ 1,206,900,000	$ 1,389,200,000	$ 1,424,100,000							$ 580,900,000	$ 858,200,000	$ 1,005,300,000																											$ 626,000,000	$ 531,000,000	$ 418,800,000			$ 44,400,000