Summary of Significant Accounting Policies	6 Months Ended
Summary of Significant Accounting Policies	Jun. 28, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies As permitted by the rules of the Securities and Exchange Commission applicable to Quarterly Reports on Form 10-Q, these notes are condensed and do not contain all disclosures required by generally accepted accounting principles (GAAP). As a result of the adoption of ASU 2016-13 "Measurement of Credit Losses on Financial Instruments," Cognex Corporation (the "Company") has provided new disclosures related to credit losses in this Quarterly Report on Form 10-Q. Reference should be made to the consolidated financial statements and related notes included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019 for a full description of other significant accounting policies. In the opinion of the management of the Company, the accompanying consolidated unaudited financial statements contain all adjustments, consisting of normal, recurring adjustments, excess and obsolete inventory charges (Note 5), intangible asset impairment charges (Note 8), restructuring charges (Note 16), and financial statement reclassifications necessary to present fairly the Company’s financial position as of June 28, 2020, and the results of its operations for the three-month and six-month periods ended June 28, 2020 and June 30, 2019, and changes in shareholders’ equity, comprehensive income, and cash flows for the periods presented. The results disclosed in the Consolidated Statements of Operations for the three-month and six-month periods ended June 28, 2020 are not necessarily indicative of the results to be expected for the full year. Cash, Cash Equivalents, and Investments Money market instruments, as well as certificates of deposit and debt securities with original maturities of three months or less, are classified as cash equivalents and are stated at amortized cost. Certificates of deposit and debt securities with original maturities greater than three months and remaining maturities of one year or less are classified as current investments. Debt securities with remaining maturities greater than one year are classified as non-current investments. It is the Company’s policy to invest in debt securities with effective maturities that do not exceed ten years. Debt securities with original maturities greater than three months are designated as available-for-sale and are reported at fair value, with unrealized gains and losses, net of tax and credit losses, recorded in shareholders’ equity as other comprehensive income (loss). Realized gains and losses are included in current operations, along with the amortization of the discount or premium on debt securities arising at acquisition, and are calculated using the specific identification method. The Company’s limited partnership interest is accounted for using the cost method because the Company’s investment is less than 5% of the partnership and the Company has no influence over the partnership’s operating and financial policies. The carrying value of this investment has been reduced to zero, and therefore, distributions are recorded as investment income as they occur. Management monitors the carrying value of its investments in debt securities compared to their fair value to determine whether a credit loss or other type of impairment has occurred. If the fair value of a debt security is less than its amortized cost, the Company assesses whether the decline has resulted from a credit loss or other factors. In assessing whether a credit loss exists, the Company compares the present value of the cash flows expected to be collected from the security with the amortized cost basis of the security. If the present value of the cash flows expected to be collected is less than the amortized cost basis of the security, then a credit loss exists and an allowance for credit losses is recorded. The allowance for credit losses is limited by the amount that the fair value is less than the amortized cost basis of the security. Credit losses on impaired securities continue to be measured in subsequent periods, using the present value of expected future cash flows, and changes in the expected credit losses are recorded in current operations. Credit losses and recoveries are included in "Other income (expense)" on the Consolidated Statement of Operations. When developing an estimate of the expected credit losses, the Company considers all available information relevant to assessing the collectability of cash flows. This information includes internal and external factors, historical and current events, reasonable and supportable forecasts including management's expectations of future economic conditions, the type of security, the credit rating of the security, the size of the loss position, as well as other relevant information. An impairment is recognized as a write-down if (i) the Company has the intent to sell the security or (ii) it is more likely than not that the Company will be required to sell the security before recovery of the entire amortized cost basis. If impairment is considered upon condition (i) or (ii) described above, the debt security's amortized cost basis is written-down to its fair value and the impairment is recognized in current operations. Subsequent increases in the fair value of the debt securities after the write-down are included in shareholders' equity as other comprehensive income. The differences between the new amortized cost basis and the cash flows expected to be collected are accreted as interest income. Unrealized losses, that have not been recorded through an allowance for credit losses or write-down of the debt security, are recorded in shareholders' equity as other comprehensive loss, net of the applicable taxes. Accounts Receivable The Company extends credit with various payment terms to customers based upon an evaluation of their financial condition. Accounts that are outstanding longer than the payment terms are considered to be past due. The Company establishes an allowance against accounts receivable for potential credit losses and records bad debt expense in current operations when it determines receivables are at risk for collection based upon the length of time the receivable has been outstanding, the customer’s current ability to pay its obligations to the Company, general economic and industry conditions, and reasonable forecasts about the future, as well as various other factors. Receivables are written off against this allowance in the period they are determined to be uncollectible and payments subsequently received on previously written-off receivables are recorded as a reversal of the bad debt expense. Credit losses are included in "Selling, general, and administrative expenses" on the Consolidated Statement of Operations.