Document and Entity Information - shares	9 Months Ended
	Oct. 02, 2015	Oct. 30, 2015
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Oct. 02, 2015
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q3
Trading Symbol	GSIG
Entity Registrant Name	GSI GROUP INC
Entity Central Index Key	0001076930
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		34,387,509