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Goodwill and Other Intangible Assets (Schedule of Intangible Assets) (Details) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]    
Finite-lived intangible assets, accumulated amortization $ (86,850,000) $ (82,025,000)
Intangible assets, gross 371,000,000 371,000,000
Intangible assets, net 284,150,000 288,975,000 [1]
Gentex Patents    
Finite-Lived Intangible Assets [Line Items]    
Finite-lived intangible assets, gross 36,352,536 34,847,029
Finite-lived intangible assets, accumulated amortization (19,760,899) (18,943,554)
Finite-lived intangible assets, net 16,591,637 15,903,475
Homelink Technology    
Finite-Lived Intangible Assets [Line Items]    
Finite-lived intangible assets, gross 180,000,000 180,000,000
Finite-lived intangible assets, accumulated amortization (67,500,000) (63,750,000)
Finite-lived intangible assets, net $ 112,500,000 $ 116,250,000
Finite-lived intangible asset, useful life 12 years 12 years
Existing Customer Platforms    
Finite-Lived Intangible Assets [Line Items]    
Finite-lived intangible assets, gross $ 43,000,000 $ 43,000,000
Finite-lived intangible assets, accumulated amortization (19,350,000) (18,275,000)
Finite-lived intangible assets, net $ 23,650,000 $ 24,725,000
Finite-lived intangible asset, useful life 10 years 10 years
Patents and Other Intangible Assets    
Finite-Lived Intangible Assets [Line Items]    
Finite-lived intangible assets, accumulated amortization $ (106,610,899) $ (100,968,554)
Intangible assets, gross 407,352,536 405,847,029
Intangible assets, net 300,741,637 304,878,475
HomeLink Trade Names and Trademarks    
Finite-Lived Intangible Assets [Line Items]    
Indefinite-lived intangible assets 52,000,000 52,000,000
Exclusive Licensing Agreement    
Finite-Lived Intangible Assets [Line Items]    
Indefinite-lived intangible assets $ 96,000,000 $ 96,000,000
[1] The condensed consolidated balance sheet at December 31, 2017 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.