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Debt and Financing Arrangements (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Oct. 01, 2014
Dec. 31, 2012
Sep. 27, 2013
Debt Instrument [Line Items]          
Amount outstanding $ 258,125,000us-gaap_LongTermDebt $ 265,625,000us-gaap_LongTermDebt      
Financial Liabilities:          
Effect of 1% increase on borrowing rate to interest expense 2,500,000gntx_EffectofOnePercentagePointIncreaseonBorrowingRatetoInterestExpense        
Interest Rate Swap          
Debt Instrument [Line Items]          
Amount of hedged item     150,000,000us-gaap_DerivativeAmountOfHedgedItem
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
   
Interest rate of derivative     1.89%us-gaap_DerivativeFixedInterestRate
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
   
Notional amount of derivative liability 150,000,000us-gaap_DerivativeLiabilityNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
0us-gaap_DerivativeLiabilityNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
150,000,000us-gaap_DerivativeLiabilityNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
0us-gaap_DerivativeLiabilityNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
 
Interest Rate Swap | Level 2          
Financial assets:          
Interest Rate Swap Asset 0us-gaap_DerivativeAssets
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
0us-gaap_DerivativeAssets
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
  0us-gaap_DerivativeAssets
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
 
Financial Liabilities:          
Interest Rate Swap Liability (Other Accrued Liabilities) 1,475,702us-gaap_DerivativeLiabilities
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
0us-gaap_DerivativeLiabilities
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
  0us-gaap_DerivativeLiabilities
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
 
Other, net          
Debt Instrument [Line Items]          
Payments of financing charges 3,500,000us-gaap_PaymentsOfFinancingCosts
/ us-gaap_IncomeStatementLocationAxis
= gntx_OtherNetMember
900,000us-gaap_PaymentsOfFinancingCosts
/ us-gaap_IncomeStatementLocationAxis
= gntx_OtherNetMember
     
LIBOR Rate          
Debt Instrument [Line Items]          
Basis spread on variable rate 1.17%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember
       
Term Loan and Revolving Facility          
Debt Instrument [Line Items]          
Extinguishment of debt amount 7,500,000us-gaap_ExtinguishmentOfDebtAmount
/ us-gaap_ExtinguishmentOfDebtAxis
= gntx_TermLoanandRevolvingFacilityMember
1,900,000us-gaap_ExtinguishmentOfDebtAmount
/ us-gaap_ExtinguishmentOfDebtAxis
= gntx_TermLoanandRevolvingFacilityMember
     
Credit Agreement          
Debt Instrument [Line Items]          
Additional principal amount available         75,000,000gntx_DebtInstrumentAdditionalBorrowingsAvailableFaceAmount
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_LineOfCreditMember
Revolving Facility          
Debt Instrument [Line Items]          
Principal amount         150,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_RevolvingCreditFacilityMember
Amount outstanding 125,000,000us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_RevolvingCreditFacilityMember
       
Available borrowing capacity 25,000,000us-gaap_DebtInstrumentUnusedBorrowingCapacityAmount
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_RevolvingCreditFacilityMember
       
Term Loan          
Debt Instrument [Line Items]          
Principal amount         150,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_LongtermDebtTypeAxis
= gntx_TermLoanMember
Amount outstanding 140,600,000us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= gntx_TermLoanMember
       
Annual principal repayments amount 7,500,000us-gaap_DebtInstrumentAnnualPrincipalPayment
/ us-gaap_LongtermDebtTypeAxis
= gntx_TermLoanMember
       
Swing Loans          
Debt Instrument [Line Items]          
Additional principal amount available         20,000,000gntx_DebtInstrumentAdditionalBorrowingsAvailableFaceAmount
/ us-gaap_LongtermDebtTypeAxis
= gntx_SwingLoansMember
Letters of Credit          
Debt Instrument [Line Items]          
Additional principal amount available         $ 20,000,000gntx_DebtInstrumentAdditionalBorrowingsAvailableFaceAmount
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_LetterOfCreditMember