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Summary of Significant Accounting and Reporting Policies (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended | 0 Months Ended
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 05, 2014
Summary Of Significant Accounting And Reporting Policies [Line Items] |  |  |  |
Allowance for obsolete and slow moving inventories | $ 6.7 gntx_AllowanceForObsoleteAndSlowMovingInventories | $ 6.9 gntx_AllowanceForObsoleteAndSlowMovingInventories |  |
Depreciation expense | 55.3 us-gaap_Depreciation | 55.0 us-gaap_Depreciation | 48.5 us-gaap_Depreciation |
Advertising and promotional costs | 1.1 us-gaap_MarketingAndAdvertisingExpense | 0.4 us-gaap_MarketingAndAdvertisingExpense | 1.0 us-gaap_MarketingAndAdvertisingExpense |
Repair and maintenance of plant and equipment | $ 17.9 us-gaap_CostOfPropertyRepairsAndMaintenance | $ 14.9 us-gaap_CostOfPropertyRepairsAndMaintenance | $ 13.8 us-gaap_CostOfPropertyRepairsAndMaintenance |
Shares related to stock plans not included in diluted average common shares outstanding because their effect would be antidilutive | 1,228,694 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount | 6,754,622 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount | 6,665,438 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount |
Common Stocks |  |  |  |
Summary Of Significant Accounting And Reporting Policies [Line Items] |  |  |  |
Common stock split conversion ratio |  |  |  | 2 us-gaap_StockholdersEquityNoteStockSplitConversionRatio1 / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember
Common stock split, stock dividend percent |  |  |  | 100.00%gntx_StockholdersEquityNoteStockSplitStockDividendPercent / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember
Buildings and improvements | Minimum |  |  |  |
Summary Of Significant Accounting And Reporting Policies [Line Items] |  |  |  |
Minimum estimated useful life, in years | 7 years |  |  |
Buildings and improvements | Maximum |  |  |  |
Summary Of Significant Accounting And Reporting Policies [Line Items] |  |  |  |
Minimum estimated useful life, in years | 30 years |  |  |
Machinery and equipment | Minimum |  |  |  |
Summary Of Significant Accounting And Reporting Policies [Line Items] |  |  |  |
Minimum estimated useful life, in years | 3 years |  |  |
Machinery and equipment | Maximum |  |  |  |
Summary Of Significant Accounting And Reporting Policies [Line Items] |  |  |  |
Minimum estimated useful life, in years | 10 years |  |  |

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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 total expense recognized in the period for promotion, public relations, and brand or product advertising. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.4) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
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