Goodwill and Other Intangible Assets (Schedule of Intangible Assets) (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, accumulated amortization	$ (154,400,000)	$ (139,925,000)
Intangible assets, gross	397,563,539	389,673,127
Intangible assets, net	243,163,539	249,748,127	[1]
Patents and Other Intangible Assets
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, accumulated amortization	(180,321,666)	(164,492,709)
Intangible assets, gross	437,034,283	428,599,404
Intangible assets, net	256,712,617	264,106,695
Gentex Patents
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	39,124,090	38,625,292
Finite-lived intangible assets, accumulated amortization	(25,858,926)	(24,528,853)
Finite-lived intangible assets, net	13,265,164	14,096,439
Vaporsens Technology Licenses
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	346,654	300,985
Finite-lived intangible assets, accumulated amortization	(62,740)	(38,856)
Finite-lived intangible assets, net	283,914	262,129
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	(120,000,000)	(108,750,000)
Finite-lived intangible assets, net	$ 60,000,000	$ 71,250,000
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	(34,400,000)	(31,175,000)
Finite-lived intangible assets, net	$ 8,600,000	$ 11,825,000
Finite-lived intangible asset, useful life	10 years	10 years
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
In-Process R&D | Vaporsens Inc
Finite-Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets	11,000,000	11,000,000
In-Process R&D | Argil, Inc.
Finite-Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets	6,278,132	6,278,132
In-Process R&D | Air-CraftGlass
Finite-Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets	800,000	$ 1,394,995
In-Process R&D | Guardian Optical Technologies
Finite-Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets	$ 8,485,407

[1] The condensed consolidated balance sheet at December 31, 2020 has been derived from the audited consolidated 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.