Goodwill and Other Intangible Assets (Schedule of Intangible Assets) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, accumulated amortization	$ (125,450,000)	$ (120,625,000)
Intangible assets, gross	371,000,000	371,000,000
Intangible assets, net	245,550,000	250,375,000	[1]
Gentex Patents
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	37,496,822	37,328,963
Finite-lived intangible assets, accumulated amortization	(23,003,239)	(22,491,010)
Finite-lived intangible assets, net	14,493,583	14,837,953
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	(97,500,000)	(93,750,000)
Finite-lived intangible assets, net	$ 82,500,000	$ 86,250,000
Finite-lived intangible asset, useful life (in years)	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	(27,950,000)	(26,875,000)
Finite-lived intangible assets, net	$ 15,050,000	$ 16,125,000
Finite-lived intangible asset, useful life (in years)	10 years	10 years
Patents and Other Intangible Assets
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, accumulated amortization	$ (148,453,239)	$ (143,116,010)
Intangible assets, gross	408,496,822	408,328,963
Intangible assets, net	260,043,583	265,212,953
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, 2019 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.