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Contingencies (Details) (Litigation Settled [Member])
 0 Months Ended
Jun. 25, 2012
co-defendant
complaint
Oct. 05, 2012
complaint
Jul. 27, 2012
complaint
Loss Contingencies [Line Items]      
Number of complaints filed 4us-gaap_LossContingencyNewClaimsFiledNumber    
Number of co-defendants 1gntx_LossContingencyNumberOfCoDefendants    
Total number of customers cited as possible co-defendants 2gntx_LossContingencyTotalNumberOfCustomersCitedAsPossibleCoDefendants    
Number of complaints served     4gntx_LossContingencyNumberOfComplaintsServed
Number of complains answered   4gntx_LossContingencyNumberOfComplaintsAnswered  
SmartBeam [Member]
      
Loss Contingencies [Line Items]      
Number of complaints filed 2us-gaap_LossContingencyNewClaimsFiledNumber
/ us-gaap_LitigationCaseAxis
= gntx_PatentInfringementSmartbeamMember
/ us-gaap_LitigationStatusAxis
= gntx_LitigationSettledMember		    
Number of patents infringed upon 1us-gaap_LossContingencyPatentsAllegedlyInfringedNumber
/ us-gaap_LitigationCaseAxis
= gntx_PatentInfringementSmartbeamMember
/ us-gaap_LitigationStatusAxis
= gntx_LitigationSettledMember		    
Monitoring System Products [Member]
      
Loss Contingencies [Line Items]      
Number of complaints filed 2us-gaap_LossContingencyNewClaimsFiledNumber
/ us-gaap_LitigationCaseAxis
= gntx_PatentInfringementMonitoringSystemProductsMember
/ us-gaap_LitigationStatusAxis
= gntx_LitigationSettledMember		    
Number of patents infringed upon 2us-gaap_LossContingencyPatentsAllegedlyInfringedNumber
/ us-gaap_LitigationCaseAxis
= gntx_PatentInfringementMonitoringSystemProductsMember
/ us-gaap_LitigationStatusAxis
= gntx_LitigationSettledMember