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Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) (Fair Value, Measurements, Recurring [Member], USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Liabilities:      
Other than temporary impairment loss $ 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet $ 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet $ 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
Other Than Temporary Impairment Losses Investments Portion Recognized In Earnings Net, non current 0cgnx_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetnoncurrent 0cgnx_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetnoncurrent 0cgnx_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetnoncurrent
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]      
Assets:      
Money market instruments 777,000cgnx_MoneyMarketInstrumentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
   
Significant Other Observable Inputs (Level 2) [Member]      
Assets:      
Corporate bonds 247,183,000cgnx_CorporateBondsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
   
Treasury bills 90,412,000cgnx_TreasuryBillsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
   
Asset-backed securities 63,867,000cgnx_AssetBackedSecuritiesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
   
Euro liquidity funds 48,235,000cgnx_Euroliquidityfunds
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
   
Agency bonds 16,449,000cgnx_AgencyBondsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
   
Sovereign bonds 13,461,000cgnx_SovereignBondsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
   
Municipal bonds 7,837,000cgnx_MunicipalBondsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
   
Supranational bonds 1,901,000cgnx_Supranationalbonds
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
   
Cash flow hedge forward contracts 108,000us-gaap_ForeignCurrencyCashFlowHedgeAssetAtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
   
Economic hedge forward contracts 6,000us-gaap_ForeignCurrencyContractAssetFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
   
Liabilities:      
Cash flow hedge forward contracts 84,000us-gaap_ForeignCurrencyCashFlowHedgeLiabilityAtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
   
Economic hedge forward contracts $ 13,000us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember