Document and Entity Information	9 Months Ended
Sep. 30, 2011	Oct. 28, 2011 Common Class A	Oct. 28, 2011 Common Class B
Entity Registrant Name	OneBeacon Insurance Group, Ltd.
Entity Central Index Key	0001369817
Document Type	10-Q
Document Period End Date	Sep. 30, 2011
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		23,313,719	71,754,738
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	Q3

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