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Goodwill and Other Intangible Assets (Schedule of Intangible Assets) (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, accumulated amortization	$ (110,975,000)	$ (101,325,000)
Intangible assets, gross	371,000,000	371,000,000
Intangible assets, net	260,025,000	269,675,000	[1]
Gentex Patents
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	37,307,011	36,737,434
Finite-lived intangible assets, accumulated amortization	(22,002,673)	(21,014,168)
Finite-lived intangible assets, net	15,304,338	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	(86,250,000)	(78,750,000)
Finite-lived intangible assets, net	$ 93,750,000	$ 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	(24,725,000)	(22,575,000)
Finite-lived intangible assets, net	$ 18,275,000	$ 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	$ (132,977,673)	$ (122,339,168)
Intangible assets, gross	408,307,011	407,737,434
Intangible assets, net	275,329,338	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.

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