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Earnings Per Share (Schedule of Earnings Per Share, Basic And Diluted) (Details) (USD $) | 3 Months Ended | 0 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 05, 2014 | Dec. 31, 2014
Numerators [Abstract] |  |  |  |
Numerator for both basic and diluted EPS, net income | $ 77,183,441 us-gaap_NetIncomeLoss | $ 68,566,071 us-gaap_NetIncomeLoss |  |
Denominators [Abstract] |  |  |  |
Denominator for basic EPS, weighted-average shares outstanding | 295,031,431 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic | 289,836,812 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic |  |
Potentially dilutive shares resulting from stock plans | 3,562,712 us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment | 3,600,202 us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment |  |
Denominator for diluted EPS | 298,594,143 us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding | 293,437,014 us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding |  |
Shares related to stock plans not included in diluted average common shares outstanding because their effect would be anti-dilutive | 1,184,635 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount | 1,188,830 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount |  |
Common Stock |  |  |  |
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items] |  |  |  |
Stock split, conversion ratio |  |  | 2 us-gaap_StockholdersEquityNoteStockSplitConversionRatio1 / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 2 us-gaap_StockholdersEquityNoteStockSplitConversionRatio1 / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember
Stock split, stock dividend |  |  | 100.00%gntx_StockholdersEquityNoteStockSplitStockDividendPercent / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 100.00%gntx_StockholdersEquityNoteStockSplitStockDividendPercent / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember

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- Definition Stockholders' Equity Note, Stock Split, Stock Dividend, Percent + References No definition available. + Details
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- Definition Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Antidilution -URI http://asc.fasb.org/extlink&oid=6505113 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Diluted Earnings Per Share -URI http://asc.fasb.org/extlink&oid=6510752 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Contingent Stock Agreement -URI http://asc.fasb.org/extlink&oid=6508534 + Details
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- Definition The portion of profit or loss for the period, net of income taxes, which is attributable to the parent. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -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 260 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Other Comprehensive Income -URI http://asc.fasb.org/extlink&oid=6519514 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Net Income -URI http://asc.fasb.org/extlink&oid=6518256 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.19) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.18) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.22) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 19 -Article 5 Reference 9: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 20 -Article 9 + Details
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- Definition The sum of dilutive potential common shares or units used in the calculation of the diluted per-share or per-unit computation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
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