The portion of reinsurance receivables representing the aggregate amount of estimated amounts due as of the balance sheet date from another insurer; also called reinsurance recoverables or unpaid losses recoverable, that are expected based upon statistical projections of incurred claims and claims settlement expenses which have not yet been reported to the insurer.

Name:	us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Third Party Reinsurance (Details) (USD $)						3 Months Ended			130 Months Ended	3 Months Ended		3 Months Ended			3 Months Ended
Third Party Reinsurance (Details) (USD $)	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2010	Mar. 31, 2012 OneBeacon contract rating	Mar. 31, 2012 OneBeacon Property catastrophe reinsurance program	Mar. 31, 2012 OneBeacon Property catastrophe reinsurance program Minimum	Mar. 31, 2012 OneBeacon Property catastrophe reinsurance program Maximum	Mar. 31, 2012 OneBeacon NICO Cover	Mar. 31, 2012 OneBeacon NICO Cover Maximum	Mar. 31, 2012 OneBeacon GRC Cover	Mar. 31, 2012 OneBeacon GRC Cover Maximum	Mar. 31, 2012 OneBeacon National Indemnity Company and General Reinsurance Corporation	Mar. 31, 2012 OneBeacon Hanover Insurance Group Ltd.	Mar. 31, 2012 OneBeacon Tokio Marine and Nichido Fire	Mar. 31, 2012 OneBeacon Tower Group Inc.	Mar. 31, 2012 OneBeacon Munich Reinsurance America	May 31, 2001 OneBeacon General Reinsurance Corporation GRC Cover	Mar. 31, 2012 Sirius group rating	Mar. 31, 2012 Sirius group General Reinsurance Corporation	Mar. 31, 2012 Sirius group Swiss Re Group	Mar. 31, 2012 Sirius group Olympus	Mar. 31, 2012 Sirius group Lloyds of London	Mar. 31, 2012 Sirius group General Insurance Company of India
Third-Party Reinsurance
Reinsurance recoverable on paid losses	$ 39,100,000	$ 30,500,000			$ 25,900,000														$ 13,200,000
Reinsurance recoverable on unpaid losses	2,465,400,000	2,507,300,000	2,304,100,000	2,344,000,000	2,269,000,000						423,700,000								356,400,000
Purchase accounting adjustments					160,000,000
Reinsurance recoverables													1,504,900,000	79,900,000	55,000,000	29,900,000	24,000,000			42,600,000	41,800,000	33,300,000	26,800,000	14,800,000
Percentage of total paid and unpaid reinsurance recoverables													66.00%	3.00%	2.00%	1.00%	1.00%			12.00%	11.00%	9.00%	7.00%	4.00%
Percentage of reinsurance recoverable that is collateralized																				1.00%	4.00%	100.00%	9.00%	2.00%
Reinsurance recoverable from various third-party reinsurers, guaranteed by the third-party reinsurer under reinsurance agreement															29,000,000
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
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															100.00%
Number of ratings in A.M. Best ratings					15														15