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Goodwill and Other Intangible Assets (Schedule of Intangible Assets) (Details) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items] |  |  |
Finite-lived intangible assets, accumulated amortization | $ (86,850,000) | $ (82,025,000) |
Intangible assets, gross | 371,000,000 | 371,000,000 |
Intangible assets, net | 284,150,000 | 288,975,000 | [1]
Gentex Patents |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |
Finite-lived intangible assets, gross | 36,352,536 | 34,847,029 |
Finite-lived intangible assets, accumulated amortization | (19,760,899) | (18,943,554) |
Finite-lived intangible assets, net | 16,591,637 | 15,903,475 |
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 | (67,500,000) | (63,750,000) |
Finite-lived intangible assets, net | $ 112,500,000 | $ 116,250,000 |
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 | (19,350,000) | (18,275,000) |
Finite-lived intangible assets, net | $ 23,650,000 | $ 24,725,000 |
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 | $ (106,610,899) | $ (100,968,554) |
Intangible assets, gross | 407,352,536 | 405,847,029 |
Intangible assets, net | 300,741,637 | 304,878,475 |
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, 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.

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