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Segment Reporting (Schedule Of Automotive And Other Segment Reporting) (Details) (USD $)
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Segment Reporting Information [Line Items]    
Revenue $ 368,937,455us-gaap_SalesRevenueGoodsNet $ 335,739,344us-gaap_SalesRevenueGoodsNet
Income from operations 107,708,506us-gaap_OperatingIncomeLoss 97,177,546us-gaap_OperatingIncomeLoss
Automotive Products    
Segment Reporting Information [Line Items]    
Revenue 360,615,289us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementBusinessSegmentsAxis
= gntx_AutomotiveProductsSegmentMember		 326,364,107us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementBusinessSegmentsAxis
= gntx_AutomotiveProductsSegmentMember
Income from operations 104,808,058us-gaap_OperatingIncomeLoss
/ us-gaap_StatementBusinessSegmentsAxis
= gntx_AutomotiveProductsSegmentMember		 93,444,468us-gaap_OperatingIncomeLoss
/ us-gaap_StatementBusinessSegmentsAxis
= gntx_AutomotiveProductsSegmentMember
Other    
Segment Reporting Information [Line Items]    
Revenue 8,322,166us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember		 9,375,237us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember
Income from operations $ 2,900,448us-gaap_OperatingIncomeLoss
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember		 $ 3,733,078us-gaap_OperatingIncomeLoss
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember