Document and Entity Information	3 Months Ended
	Apr. 05, 2015
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	Apr. 05, 2015
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Common Stock, Shares Outstanding	87,128,037