Third Party Reinsurance (Details) (USD $)										3 Months Ended	9 Months Ended	122 Months Ended		1 Months Ended	3 Months Ended	9 Months Ended		3 Months Ended	9 Months Ended			6 Months Ended	9 Months Ended
	Sep. 30, 2011 rating state	Jun. 30, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Dec. 31, 2009	Sep. 30, 2011 OneBeacon	May 31, 2001 OneBeacon contract	Sep. 30, 2011 OneBeacon National Indemnity Company and General Reinsurance Corporation	Sep. 30, 2011 OneBeacon National Indemnity Company NICO Cover	Sep. 30, 2011 OneBeacon National Indemnity Company NICO Cover	Jun. 30, 2011 OneBeacon National Indemnity Company NICO Cover	Dec. 31, 2007 OneBeacon National Indemnity Company NICO Cover	May 31, 2001 OneBeacon General Reinsurance Corporation GRC Cover	Sep. 30, 2011 OneBeacon General Reinsurance Corporation GRC Cover	Sep. 30, 2011 OneBeacon General Reinsurance Corporation GRC Cover	Sep. 30, 2011 OneBeacon Tower Insurance Company	Sep. 30, 2011 OneBeacon Tokio Marine and Nichido Fire	Sep. 30, 2011 OneBeacon Tokio Marine and Nichido Fire	Sep. 30, 2011 OneBeacon Hanover Insurance Company	Sep. 30, 2011 OneBeacon Munich Reinsurance America	Jun. 30, 2010 OneBeacon Northeast homeowners business	Sep. 30, 2011 OneBeacon Northeast homeowners business	Sep. 30, 2011 OneBeacon Property catastrophe reinsurance program	Sep. 30, 2011 Sirius Group.	Sep. 30, 2011 Sirius Group. General Reinsurance Corporation	Sep. 30, 2011 Sirius Group. Lloyds of London	Sep. 30, 2011 Sirius Group. Olympus	Sep. 30, 2011 Sirius Group. Swiss Re Group	Sep. 30, 2011 Sirius Group. Michigan Catastrophic Claims Association
Third party reinsurance
Reinsurance recoverable on paid losses	$ 30,900,000		$ 63,100,000				$ 13,700,000																		$ 17,200,000
Reinsurance recoverable on unpaid losses	2,581,200,000	2,193,000,000	2,344,000,000	2,592,100,000	2,748,300,000	2,771,500,000	2,405,700,000																		342,100,000
Purchase accounting adjustments							166,600,000
Percentage of total paid and unpaid reinsurance recoverables (as a percent)									65.00%								2.00%	2.00%	2.00%	5.00%	1.00%					12.00%	9.00%	11.00%	11.00%	4.00%
Reinsurance recoverables									1,565,200,000								53,700,000	55,800,000	55,800,000	110,100,000	31,500,000					43,300,000	31,300,000	40,800,000	40,000,000	14,600,000
Third party recoverables 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																		29,300,000	29,300,000
The percentage ceded under quota share reinsurance agreement (as a percent)																		100.00%	100.00%				30.00%
Number of reinsurance contracts the seller caused OneBeacon to purchase immediately prior to the entity's acquisition of OneBeacon								2
Maximum cover under reinsurance contract										2,500,000,000	2,500,000,000			570,000,000
Adverse loss reserve development on losses occurring in years 2000 and prior														400,000,000
Reserves ceded as of the date of the OneBeacon acquisition														170,000,000
Utilized coverage from NICO												2,300,000,000
Reinsurance receivables paid by the reinsurer												1,400,000,000			13,600,000	60,700,000
Percentage of utilized coverage from NICO that relates to uncollectible Third Party Recoverables (as a percent)												8.00%
Utilized coverage from NICO that relates to uncollectible Third Party Recoverables												187,700,000
Ceded estimated incurred losses										2,300,000,000	2,300,000,000		2,200,000,000
Maximum 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 (as a percent)																								26.00%
Amount of loss on a single catastrophe reinsured, first retention slab for second layer, low end of range																								50,000,000
Amount of loss on a single catastrophe reinsured, first retention slab for second layer, high end of range																								100,000,000
Second retention percentage of loss on a single catastrophe reinsured, second layer (as a percent)																								10.00%
Amount of loss on a single catastrophe reinsured, second retention slab for second layer, low end of range																								100,000,000
Amount of loss on a single catastrophe reinsured, second retention slab for second layer, high end of range																								175,000,000
Minimum amount of loss on a single catastrophe retained, third layer																								225,000,000
Premiums ceded under quota share agreement																						$ 25,600,000
Percentage collateralized (as a percent)																										2.00%	6.00%	100.00%	4.00%
Number of ratings in A.M. Best ratings	15