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Goodwill and Other Intangible Assets (Schedule of Intangible Assets) (Details) (USD $)
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Schedule of Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]    
Intangible assets, net 342,050,000us-gaap_IntangibleAssetsNetExcludingGoodwill 346,875,000us-gaap_IntangibleAssetsNetExcludingGoodwill [1]
Gentex Patents    
Schedule of Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]    
Finite-lived intangible assets, gross 30,363,189us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_PatentsMember		 30,132,116us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_PatentsMember
Finite-lived intangible assets, accumulated amortization (11,713,249)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_PatentsMember		 (11,065,153)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_PatentsMember
Finite-lived intangible assets, net 18,649,940us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_PatentsMember		 19,066,963us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_PatentsMember
Patents and Other Intangible Assets [Member]    
Schedule of Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]    
Finite-lived intangible assets, accumulated amortization (40,663,249)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= gntx_PatentsandOtherIntangibleAssetsMember		 (35,190,153)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= gntx_PatentsandOtherIntangibleAssetsMember
Intangible assets, gross 401,363,189us-gaap_IntangibleAssetsGrossExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= gntx_PatentsandOtherIntangibleAssetsMember		 401,132,116us-gaap_IntangibleAssetsGrossExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= gntx_PatentsandOtherIntangibleAssetsMember
Intangible assets, net 360,699,940us-gaap_IntangibleAssetsNetExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= gntx_PatentsandOtherIntangibleAssetsMember		 365,941,963us-gaap_IntangibleAssetsNetExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= gntx_PatentsandOtherIntangibleAssetsMember
HomeLink®    
Schedule of Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]    
Finite-lived intangible assets, accumulated amortization (28,950,000)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember		 (24,125,000)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
Intangible assets, gross 371,000,000us-gaap_IntangibleAssetsGrossExcludingGoodwill
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember		 371,000,000us-gaap_IntangibleAssetsGrossExcludingGoodwill
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
Intangible assets, net 342,050,000us-gaap_IntangibleAssetsNetExcludingGoodwill
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember		 346,875,000us-gaap_IntangibleAssetsNetExcludingGoodwill
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
HomeLink® | Homelink Technology    
Schedule of Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]    
Finite-lived intangible assets, gross 180,000,000us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_DevelopedTechnologyRightsMember		 180,000,000us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_DevelopedTechnologyRightsMember
Finite-lived intangible assets, accumulated amortization (22,500,000)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_DevelopedTechnologyRightsMember		 (18,750,000)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_DevelopedTechnologyRightsMember
Finite-lived intangible assets, net 157,500,000us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_DevelopedTechnologyRightsMember		 161,250,000us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_DevelopedTechnologyRightsMember
HomeLink® | Homelink Technology | Minimum [Member]    
Schedule of Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]    
Finite-lived intangible asset, useful life 12 years 0 days 12 years
HomeLink® | Existing Customer Platforms    
Schedule of Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]    
Finite-lived intangible assets, gross 43,000,000us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerContractsMember		 43,000,000us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerContractsMember
Finite-lived intangible assets, accumulated amortization (6,450,000)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerContractsMember		 (5,375,000)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerContractsMember
Finite-lived intangible assets, net 36,550,000us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerContractsMember		 37,625,000us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerContractsMember
Finite-lived intangible asset, useful life 10 years 10 years
HomeLink® | HomeLink Trade Names and Trademarks    
Schedule of Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]    
Indefinite-lived intangible assets 52,000,000us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_TrademarksAndTradeNamesMember		 52,000,000us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_TrademarksAndTradeNamesMember
HomeLink® | Exclusive Licensing Agreement    
Schedule of Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]    
Indefinite-lived intangible assets 96,000,000us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_LicensingAgreementsMember		 96,000,000us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_LicensingAgreementsMember
[1] Note: The condensed consolidated balance sheet at December 31, 2014 has been derived from the audited financial statements at that date but does not include all of the information and footnotes required by accounting principles generally accepted in the United States for complete financial statements.