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Segment Reporting (Schedule of Automotive Customers Individually Accounted For 10% or More of Net Sales) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenue, Major Customer [Line Items]
Percentage net sales invoiced and paid in foreign currencies	6.00%gntx_PercentageNetSalesInvoicedAndPaidInForeignCurrencies	7.00%gntx_PercentageNetSalesInvoicedAndPaidInForeignCurrencies	8.00%gntx_PercentageNetSalesInvoicedAndPaidInForeignCurrencies
Number of customers which individually accounted for 10% or more of net sales	4gntx_NumberOfCustomersWhichIndividuallyAccountedFor10PercentOrMoreOfNetSales
Percentage of net sales five automotive customers individually accounted for	10.00%gntx_PercentageOfNetSalesByAutomotiveCustomers
Customer concentration risk | Toyota Motor Corporation
Revenue, Major Customer [Line Items]
Percentage of revenue individually accounted by customers (less than 10% in 2014 and 2013 for Hyundai/Kia and General Motors and less than 10% for FCA Group in 2013 and 2012)	12.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_ToyotaMotorCorporationMember	13.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_ToyotaMotorCorporationMember	14.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_ToyotaMotorCorporationMember
Customer concentration risk | VW/Audi
Revenue, Major Customer [Line Items]
Percentage of revenue individually accounted by customers (less than 10% in 2014 and 2013 for Hyundai/Kia and General Motors and less than 10% for FCA Group in 2013 and 2012)	14.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_VwAudiMember	14.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_VwAudiMember	14.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_VwAudiMember
Customer concentration risk | Hyundai/Kia
Revenue, Major Customer [Line Items]
Percentage of revenue individually accounted by customers (less than 10% in 2014 and 2013 for Hyundai/Kia and General Motors and less than 10% for FCA Group in 2013 and 2012)	10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_HyundaiKiaMember	10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_HyundaiKiaMember	11.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_HyundaiKiaMember
Customer concentration risk | Daimler AG
Revenue, Major Customer [Line Items]
Percentage of revenue individually accounted by customers (less than 10% in 2014 and 2013 for Hyundai/Kia and General Motors and less than 10% for FCA Group in 2013 and 2012)	10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_DaimlerAgMember	11.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_DaimlerAgMember	11.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_DaimlerAgMember
Customer concentration risk | General Motors
Revenue, Major Customer [Line Items]
Percentage of revenue individually accounted by customers (less than 10% in 2014 and 2013 for Hyundai/Kia and General Motors and less than 10% for FCA Group in 2013 and 2012)	10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_GeneralMotorsMember	10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_GeneralMotorsMember	10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_GeneralMotorsMember
Customer concentration risk | FCA Group [Member]
Revenue, Major Customer [Line Items]
Percentage of revenue individually accounted by customers (less than 10% in 2014 and 2013 for Hyundai/Kia and General Motors and less than 10% for FCA Group in 2013 and 2012)	11.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_FCAGroupMember	10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_FCAGroupMember	10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = gntx_FCAGroupMember

X

- Definition

Number of customers which individually accounted for 10% or more of net sales.

+ References

No definition available.

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- Definition

Percentage net sales invoiced and paid in foreign currencies.

+ References

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- Definition

Percentage Of Net Sales By Automotive Customers

+ References

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X

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 21

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13537-108611



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

-SubTopic 10

-Section 50

-Paragraph 20

-URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6404-108592



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 20

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13531-108611



Reference 4: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Standards Codification

-Topic 275

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-Section 50

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