Third Party Reinsurance (Details) (USD $)									6 Months Ended				134 Months Ended	6 Months Ended	134 Months Ended					6 Months Ended
	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Nov. 17, 2006	Jun. 30, 2012 OneBeacon Contract Rating	Jun. 30, 2012 OneBeacon Property catastrophe reinsurance program	Jun. 30, 2012 OneBeacon Property catastrophe reinsurance program Minimum	Jun. 30, 2012 OneBeacon Property catastrophe reinsurance program Maximum	Jun. 30, 2012 OneBeacon NICO Cover	Jun. 30, 2012 OneBeacon NICO Cover	Jun. 30, 2012 OneBeacon GRC Cover	Jun. 30, 2012 OneBeacon GRC Cover	Jun. 30, 2012 OneBeacon National Indemnity Company and General Reinsurance Corporation		Jun. 30, 2012 OneBeacon National Indemnity Company and General Reinsurance Corporation NICO Cover	Jun. 30, 2012 OneBeacon Hanover Insurance Group Ltd.	Jun. 30, 2012 OneBeacon Tokio Marine and Nichido Fire		Jun. 30, 2012 OneBeacon Tower Group Inc.	Jun. 30, 2012 OneBeacon Munich Reinsurance America	May 31, 2001 OneBeacon General Reinsurance Corporation GRC Cover	Jun. 30, 2012 Sirius group Rating	Jun. 30, 2012 Sirius group General Reinsurance Corporation	Jun. 30, 2012 Sirius group Swiss Re Group	Jun. 30, 2012 Sirius group Olympus		Jun. 30, 2012 Sirius group Lloyds of London	Jun. 30, 2012 Sirius group Michigan Catastorphic Claims Association [Member]
Third-Party Reinsurance
Stock Repurchase Program, Number of Shares Authorized to be Repurchased							1,000,000
Reinsurance recoverable on paid losses	$ 39,700,000		$ 30,500,000					$ 24,200,000																	$ 15,500,000
Reinsurance recoverable on unpaid losses	2,369,400,000	2,465,400,000	2,507,300,000	2,193,100,000	2,304,200,000	2,344,000,000		2,197,400,000						409,300,000	409,300,000										328,700,000
Purchase accounting adjustments								156,700,000
Reinsurance recoverables																1,475,100,000	[1]		74,800,000	54,400,000	[2]	26,200,000	25,500,000			42,300,000	39,700,000	29,500,000	[3]	25,200,000	14,600,000
Percentage of total paid and unpaid reinsurance recoverables																66.00%	[1]		3.00%	2.00%	[2]	1.00%	1.00%			12.00%	12.00%	9.00%	[3]	7.00%	4.00%
Percentage of reinsurance recoverable that is collateralized																										1.00%	5.00%	100.00%	[3]	7.00%	0.00%
Property Catastrophe Reinsurance Program Second Layer First Retention Percentage									55.00%
Second retention percentage of loss on a single catastrophe reinsured, second layer									15.00%
Property Catastrophe Reinsurance Program Second Layer Third Retention Percentage									10.00%
Reinsurance recoverable from various third-party reinsurers, guaranteed by the third-party reinsurer under reinsurance agreement																				28,300,000
Number of reinsurance contracts the seller caused OneBeacon to purchase immediately prior to the entity's acquisition of OneBeacon								2
Cover under reinsurance contract												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
Reinsurance receivables paid by the reinsurer													2,300,000,000
Policyholder Benefits and Claims Incurred Ceded															562,000,000
Amount of loss on a single catastrophe retained, first layer									25,000,000
Amount of loss on a single catastrophe reinsured, second layer									155,000,000
Amount of loss on a single catastrophe reinsured, first retention slab for second layer										25,000,000	40,000,000
Amount of loss on a single catastrophe reinsured, second retention slab for second layer										40,000,000	80,000,000
Property Catastrophe Reinsurance Program Third Retention Slab For Second Layer										80,000,000	180,000,000
Amount of loss on a single catastrophe retained, third layer									180,000,000
The percentage ceded under quota share reinsurance agreement																				100.00%
Number of ratings in A.M. Best ratings								15																	15
Third Party Recoverables Reinsurer would Pay if Uncollected from Third Party Reinsurer																		$ 198,300,000

[1] (2) Includes $198.3 of Third Party Recoverables (as defined below), which NICO (as defined below) would pay under the terms of the NICO Cover (as defined below) if they are unable to collect from third party reinsurers.

[2] (3) Includes $28.3 of reinsurance recoverables from various third party reinsurers that are guaranteed by Tokio Marine and Nichido Fire under the terms of a 100% quota share reinsurance agreement between Houston General Insurance Company and Tokio Marine and Nichido Fire.

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