Document and Entity Information	9 Months Ended
	Sep. 28, 2014
Document And Entity Information [Abstract]
Entity Registrant Name	COGNEX CORP
Entity Central Index Key	0000851205
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-Q
Document Period End Date	Sep. 28, 2014
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	Q3
Amendment Flag	false
Entity Common Stock, Shares Outstanding	87,045,169