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Stock-Based Compensation Plans (Details) (USD $) | 12 Months Ended | 36 Months Ended | 0 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2014 | Dec. 05, 2014 | May 31, 2014 | Dec. 31, 2008 | Dec. 31, 2003
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Shares sold to employees during period | 137,028 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans | 155,264 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans | 204,996 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans |  |  |  |  |
Shares granted with restriction period | 431,880 us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross | 374,180 us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross | 451,460 us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross |  |  |  |  |
Employee Stock Option Plan |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Options available for grant |  |  |  |  |  | 24,000,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockOptionPlanMember |  |
Options granted net of shares from canceled/expired options | 5,255,536 gntx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrants / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockOptionPlanMember |  |  |  |  |  |  |
Unrecognized compensation cost | 27,959,369 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockOptionPlanMember |  |  | 27,959,369 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockOptionPlanMember |  |  |  |
Weighted-average period for unrecognized compensation cost expected to be recognized | 2 years 1 month 6 days |  |  |  |  |  |  |
Non-Employee Director Stock Option Plan |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Options granted net of shares from canceled/expired options |  |  |  | 1,086,480 gntx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrants / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_NonEmployeeDirectorStockOptionPlanMember |  |  |  |
Option vesting period | 6 months |  |  |  |  |  |  |
Shares approved |  |  | 1,000,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_NonEmployeeDirectorStockOptionPlanMember |  |  |  |  |
Option expiration period, years | 10 years |  |  |  |  |  |  |
Previous Non-Employee Director Stock Option Plan |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Options granted net of shares from canceled/expired options |  |  |  | 266,000 gntx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrants / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_PreviousNonEmployeeDirectorStockOptionPlanMember |  |  |  |
Restricted Stock Plan |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Shares approved |  |  |  |  |  |  | 4,000,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_RestrictedStockPlanMember |
Restricted period, maximum, in years | 10 years |  |  |  |  |  |  |
Shares granted with restriction period | 1,577,580 us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_RestrictedStockPlanMember |  |  |  |  |  |  |
Restricted period for granted shares, in years | 5 years |  |  |  |  |  |  |
Unearned stock-based compensation | 12,327,280 us-gaap_DeferredCompensationEquity / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_RestrictedStockPlanMember |  |  | 12,327,280 us-gaap_DeferredCompensationEquity / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_RestrictedStockPlanMember |  |  |  |
Amortization expense | 3,095,136 us-gaap_RestrictedStockExpense / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_RestrictedStockPlanMember | 2,760,446 us-gaap_RestrictedStockExpense / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_RestrictedStockPlanMember | 2,108,583 us-gaap_RestrictedStockExpense / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_RestrictedStockPlanMember |  |  |  |  |
Employee Stock Purchase Plan |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Shares approved |  |  |  |  |  |  |  | 2,000,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockPurchasePlanMember
ESPP discount rate | 85.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPricePurchaseDate / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockPurchasePlanMember |  |  |  |  |  |  |
Discount recognized as compensation expense |  | 15.00%gntx_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountRecognizedAsCompensationExpense / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockPurchasePlanMember |  |  |  |  |  |
Employee Stock Purchase Plan, 2003 |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Shares sold to employees during period | 0 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockPurchasePlan2003Member | 52,340 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockPurchasePlan2003Member | 204,996 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockPurchasePlan2003Member |  |  |  |  |
Cumulative shares sold to employees during period | 2,400,000 gntx_CumulativeStockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockPurchasePlan2003Member |  |  | 2,400,000 gntx_CumulativeStockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockPurchasePlan2003Member |  |  |  |
Weighted average fair value of shares sold | 8.51 gntx_StockIssuedDuringPeriodWeightedAverageFairValueEmployeeStockPurchasePlans / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockPurchasePlan2003Member |  |  | 8.51 gntx_StockIssuedDuringPeriodWeightedAverageFairValueEmployeeStockPurchasePlans / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockPurchasePlan2003Member |  |  |  |
Employee Stock Purchase Plan, 2013 |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Shares sold to employees during period | 137,028 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockPurchasePlan2013Member | 102,924 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockPurchasePlan2013Member | 0 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockPurchasePlan2013Member |  |  |  |  |
Cumulative shares sold to employees during period | 239,952 gntx_CumulativeStockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockPurchasePlan2013Member |  |  | 239,952 gntx_CumulativeStockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockPurchasePlan2013Member |  |  |  |
Weighted average fair value of shares sold | 12.99 gntx_StockIssuedDuringPeriodWeightedAverageFairValueEmployeeStockPurchasePlans / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockPurchasePlan2013Member |  |  | 12.99 gntx_StockIssuedDuringPeriodWeightedAverageFairValueEmployeeStockPurchasePlans / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_EmployeeStockPurchasePlan2013Member |  |  |  |
Minimum | Employee Stock Option Plan |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Option vesting period | 1 year |  |  |  |  |  |  |
Award expiration period | 5 years |  |  |  |  |  |  |
Minimum | Restricted Stock Plan |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Market price | 13.39 gntx_ShareBasedCompensationArrangementByShareBasedPaymentAwardMarketPriceOfRestrictedStock / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_RestrictedStockPlanMember / us-gaap_RangeAxis = us-gaap_MinimumMember | 10.12 gntx_ShareBasedCompensationArrangementByShareBasedPaymentAwardMarketPriceOfRestrictedStock / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_RestrictedStockPlanMember / us-gaap_RangeAxis = us-gaap_MinimumMember | 8.64 gntx_ShareBasedCompensationArrangementByShareBasedPaymentAwardMarketPriceOfRestrictedStock / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_RestrictedStockPlanMember / us-gaap_RangeAxis = us-gaap_MinimumMember |  |  |  |  |
Maximum | Employee Stock Option Plan |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Option vesting period | 5 years |  |  |  |  |  |  |
Award expiration period | 7 years |  |  |  |  |  |  |
Maximum | Restricted Stock Plan |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Market price | 18.31 gntx_ShareBasedCompensationArrangementByShareBasedPaymentAwardMarketPriceOfRestrictedStock / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_RestrictedStockPlanMember / us-gaap_RangeAxis = us-gaap_MaximumMember | 16.42 gntx_ShareBasedCompensationArrangementByShareBasedPaymentAwardMarketPriceOfRestrictedStock / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_RestrictedStockPlanMember / us-gaap_RangeAxis = us-gaap_MaximumMember | 12.23 gntx_ShareBasedCompensationArrangementByShareBasedPaymentAwardMarketPriceOfRestrictedStock / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = gntx_RestrictedStockPlanMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |  |
Stock Compensation Plan [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Number of plans | 4 gntx_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofPlans / us-gaap_AwardTypeAxis = us-gaap_StockCompensationPlanMember |  |  | 4 gntx_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofPlans / us-gaap_AwardTypeAxis = us-gaap_StockCompensationPlanMember |  |  |  |
Employee Stock Option [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Number of plans | 2 gntx_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofPlans / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember |  |  | 2 gntx_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofPlans / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember |  |  |  |
Common Stocks |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [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 |  |  |

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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Discount rate from fair value on purchase date that participants pay for shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Ratio applied to the conversion of stock split, for example but not limited to, one share converted to two or two shares converted to one. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Stock Split -URI http://asc.fasb.org/extlink&oid=6525746 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SAB TOPIC 4.C) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187143-122770 + Details
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- Definition Number of shares issued during the period as a result of an employee stock purchase plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30 -Article 5 + Details
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- Definition Total number of shares issued during the period, including shares forfeited, as a result of Restricted Stock Awards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
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