Document and Entity Information	6 Months Ended
	Jun. 29, 2012	Jul. 30, 2012
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 29, 2012
Document Fiscal Year Focus	2012
Document Fiscal Period Focus	Q2
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		33,702,870