Document and Entity Information	9 Months Ended
	Sep. 30, 2012	Oct. 26, 2012 Class A common	Oct. 26, 2012 Class B common
Entity Registrant Name	OneBeacon Insurance Group, Ltd.
Entity Central Index Key	0001369817
Document Type	10-Q
Document Period End Date	Sep. 30, 2012
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		23,633,774	71,754,738
Document Fiscal Year Focus	2012
Document Fiscal Period Focus	Q3