Document and Entity Information	9 Months Ended
	Oct. 01, 2017 shares
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	Oct. 01, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q3
Amendment Flag	false
Entity Common Stock, Shares Outstanding	86,586,599