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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 $ 6.7gntx_AllowanceForObsoleteAndSlowMovingInventories $ 6.9gntx_AllowanceForObsoleteAndSlowMovingInventories    
Depreciation expense 55.3us-gaap_Depreciation 55.0us-gaap_Depreciation 48.5us-gaap_Depreciation  
Advertising and promotional costs 1.1us-gaap_MarketingAndAdvertisingExpense 0.4us-gaap_MarketingAndAdvertisingExpense 1.0us-gaap_MarketingAndAdvertisingExpense  
Repair and maintenance of plant and equipment $ 17.9us-gaap_CostOfPropertyRepairsAndMaintenance $ 14.9us-gaap_CostOfPropertyRepairsAndMaintenance $ 13.8us-gaap_CostOfPropertyRepairsAndMaintenance  
Shares related to stock plans not included in diluted average common shares outstanding because their effect would be antidilutive 1,228,694us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount 6,754,622us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount 6,665,438us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount  
Common Stocks        
Summary Of Significant Accounting And Reporting Policies [Line Items]        
Common stock split conversion ratio       2us-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