| Document and Entity Information | Feb. 11, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Feb. 11, 2021 |
| Entity Registrant Name | Cognex Corporation |
| Entity Incorporation, State or Country Code | MA |
| Entity File Number | 001-34218 |
| Entity Tax Identification Number | 04-2713778 |
| Entity Address, Address Line One | One Vision Drive |
| Entity Address, City or Town | Natick |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 01760-2059 |
| City Area Code | 508 |
| Local Phone Number | 650-3000 |
| Entity Information, Former Legal or Registered Name | N/A |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000851205 |
| Title of 12(b) Security | Common Stock, par value $.002 per share |
| Trading Symbol | CGNX |
| Security Exchange Name | NASDAQ |
