Goodwill and Other Intangible Assets (Narrative) (Details) - USD ($)	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	[1]	Sep. 27, 2013
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 307,365,845		$ 307,365,845
Amortization expense, 2018	22,000,000
Amortization expense, 2019	22,000,000
Amortization expense, 2020	22,000,000
Amortization expense, 2021	22,000,000
Amortization expense, 2022	22,000,000
HomeLink®
Finite-Lived Intangible Assets [Line Items]
Goodwill					$ 307,400,000
Gentex Patents
Finite-Lived Intangible Assets [Line Items]
Amortization expense	5,600,000	$ 5,700,000
Other Intangible Assets
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ 11,300,000	$ 11,300,000

[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.3