Goodwill and Other Intangible Assets (Schedule of Intangible Assets) (Details) - USD ($)	3 Months Ended	9 Months Ended
	Mar. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, accumulated amortization		$ (115,800,000)	$ (101,325,000)
Intangible assets, gross		371,000,000	371,000,000
Intangible assets, net		255,200,000	269,675,000	[1]
Gentex Patents
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross		37,160,431	36,737,434
Finite-lived intangible assets, accumulated amortization		(22,094,953)	(21,014,168)
Finite-lived intangible assets, net		15,065,478	15,723,266
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		(90,000,000)	(78,750,000)
Finite-lived intangible assets, net		$ 90,000,000	101,250,000
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		(25,800,000)	(22,575,000)
Finite-lived intangible assets, net		$ 17,200,000	20,425,000
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		$ (137,894,953)	(122,339,168)
Intangible assets, gross		408,160,431	407,737,434
Intangible assets, net		270,265,478	285,398,266
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, 2018 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.