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Goodwill and Other Intangible Assets (Details) (USD $) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Sep. 27, 2013
Schedule of Finite-Lived and Indefinite-Lived Intangible Assets [Line Items] |  |  |  |
Goodwill | $ 307,365,845 us-gaap_Goodwill | $ 307,365,845 us-gaap_Goodwill |  |
Percentage of fair value in excess of carrying amount for automotive reporting unit | 248.00%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount |  |  |
Amortization expense | 22,000,000 us-gaap_AmortizationOfIntangibleAssets | 7,900,000 us-gaap_AmortizationOfIntangibleAssets | 1,700,000 us-gaap_AmortizationOfIntangibleAssets |
Patents and Other Intangible Assets [Member] |  |  |  |
Schedule of Finite-Lived and Indefinite-Lived Intangible Assets [Line Items] |  |  |  |
Finite-lived intangible asset, useful life | 10 years |  |  |
Accumulated amortization | 35,200,000 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = gntx_PatentsandOtherIntangibleAssetsMember | 13,400,000 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = gntx_PatentsandOtherIntangibleAssetsMember |  |
Amortization expense, 2015 | 22,000,000 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = gntx_PatentsandOtherIntangibleAssetsMember |  |  |
Amortization expense, 2016 | 22,000,000 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = gntx_PatentsandOtherIntangibleAssetsMember |  |  |
Amortization expense, 2017 | 22,000,000 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = gntx_PatentsandOtherIntangibleAssetsMember |  |  |
Amortization expense, 2018 | 22,000,000 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = gntx_PatentsandOtherIntangibleAssetsMember |  |  |
Amortization expense, 2019 | 22,000,000 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = gntx_PatentsandOtherIntangibleAssetsMember |  |  |
HomeLink® |  |  |  |
Schedule of Finite-Lived and Indefinite-Lived Intangible Assets [Line Items] |  |  |  |
Goodwill | 307,365,845 us-gaap_Goodwill / us-gaap_BusinessAcquisitionAxis = gntx_HomeLinkMember |  |  | 337,600,000 us-gaap_Goodwill / us-gaap_BusinessAcquisitionAxis = gntx_HomeLinkMember
Accumulated amortization | $ 24,125,000 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_BusinessAcquisitionAxis = gntx_HomeLinkMember | $ 4,825,000 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_BusinessAcquisitionAxis = gntx_HomeLinkMember |  |

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- Definition The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275 + Details
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- Definition Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275 + Details
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- Definition Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275 + Details
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- Definition Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275 + Details
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