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

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