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Segment Reporting (Schedule Of Automotive Customers Individually Accounted For 10% Or More Of Net Sales) (Details) | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Revenue, Major Customer [Line Items] |  |  |  |
Percentage net sales invoiced and paid in foreign currencies | 8.00% |  |  |
Number of customers which individually accounted for 10% or more of net sales | 5 |  |  |
Percentage of net sales five automotive customers individually accounted for | 10.00% |  |  |
Customer [Member] | Toyota Motor Corporation [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Percentage of revenue individually accounted by customers | 14.00% | 12.00% | 15.00% |
Customer [Member] | VW/Audi [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Percentage of revenue individually accounted by customers | 14.00% | 15.00% | 13.00% |
Customer [Member] | Hyundai/Kia [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Percentage of revenue individually accounted by customers | 11.00% | 11.00% | 0.00% | [1]
Customer [Member] | Daimler AG [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Percentage of revenue individually accounted by customers | 11.00% | 11.00% | 12.00% |
Customer [Member] | General Motors [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Percentage of revenue individually accounted by customers | 10.00% | 12.00% | 15.00% |
[1] | # Less than 10%.

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- Definition Number of customers which individually accounted for 10% or more of net sales. + References No definition available. + Details
Name: | gntx_NumberOfCustomersWhichIndividuallyAccountedFor10PercentOrMoreOfNetSales
Namespace Prefix: | gntx_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition Percentage net sales invoiced and paid in foreign currencies. + References No definition available. + Details
Name: | gntx_PercentageNetSalesInvoicedAndPaidInForeignCurrencies
Namespace Prefix: | gntx_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Percentage Of Net Sales By Automotive Customers + References No definition available. + Details
Name: | gntx_PercentageOfNetSalesByAutomotiveCustomers
Namespace Prefix: | gntx_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | us-gaap_EntityWideRevenueMajorCustomerLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of revenue generated from a single external customer that accounts for 10 percent or more of an entity's revenues. + References No definition available. + Details
Name: | us-gaap_EntityWideRevenueMajorCustomerPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration