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Segment Reporting (Schedule of Automotive Customers Individually Accounted For 10% or More of Net Sales) (Details) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Revenue, Major Customer [Line Items] |  |  |
Percentage net sales invoiced and paid in foreign currencies | 6.00%gntx_PercentageNetSalesInvoicedAndPaidInForeignCurrencies | 7.00%gntx_PercentageNetSalesInvoicedAndPaidInForeignCurrencies | 8.00%gntx_PercentageNetSalesInvoicedAndPaidInForeignCurrencies
Number of customers which individually accounted for 10% or more of net sales | 4 gntx_NumberOfCustomersWhichIndividuallyAccountedFor10PercentOrMoreOfNetSales |  |
Percentage of net sales five automotive customers individually accounted for | 10.00%gntx_PercentageOfNetSalesByAutomotiveCustomers |  |
Customer concentration risk | Toyota Motor Corporation |  |  |
Revenue, Major Customer [Line Items] |  |  |
Percentage of revenue individually accounted by customers (less than 10% in 2014 and 2013 for Hyundai/Kia and General Motors and less than 10% for FCA Group in 2013 and 2012) | 12.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_ToyotaMotorCorporationMember | 13.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_ToyotaMotorCorporationMember | 14.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_ToyotaMotorCorporationMember
Customer concentration risk | VW/Audi |  |  |
Revenue, Major Customer [Line Items] |  |  |
Percentage of revenue individually accounted by customers (less than 10% in 2014 and 2013 for Hyundai/Kia and General Motors and less than 10% for FCA Group in 2013 and 2012) | 14.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_VwAudiMember | 14.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_VwAudiMember | 14.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_VwAudiMember
Customer concentration risk | Hyundai/Kia |  |  |
Revenue, Major Customer [Line Items] |  |  |
Percentage of revenue individually accounted by customers (less than 10% in 2014 and 2013 for Hyundai/Kia and General Motors and less than 10% for FCA Group in 2013 and 2012) | 10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_HyundaiKiaMember | 10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_HyundaiKiaMember | 11.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_HyundaiKiaMember
Customer concentration risk | Daimler AG |  |  |
Revenue, Major Customer [Line Items] |  |  |
Percentage of revenue individually accounted by customers (less than 10% in 2014 and 2013 for Hyundai/Kia and General Motors and less than 10% for FCA Group in 2013 and 2012) | 10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_DaimlerAgMember | 11.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_DaimlerAgMember | 11.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_DaimlerAgMember
Customer concentration risk | General Motors |  |  |
Revenue, Major Customer [Line Items] |  |  |
Percentage of revenue individually accounted by customers (less than 10% in 2014 and 2013 for Hyundai/Kia and General Motors and less than 10% for FCA Group in 2013 and 2012) | 10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_GeneralMotorsMember | 10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_GeneralMotorsMember | 10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_GeneralMotorsMember
Customer concentration risk | FCA Group [Member] |  |  |
Revenue, Major Customer [Line Items] |  |  |
Percentage of revenue individually accounted by customers (less than 10% in 2014 and 2013 for Hyundai/Kia and General Motors and less than 10% for FCA Group in 2013 and 2012) | 11.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_FCAGroupMember | 10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_FCAGroupMember | 10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_FCAGroupMember

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- Definition Number of customers which individually accounted for 10% or more of net sales. + References No definition available. + Details
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- Definition Percentage net sales invoiced and paid in foreign currencies. + References No definition available. + Details
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