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Earnings Per Share (Schedule of Earnings Per Share, Basic And Diluted) (Details) (USD $)
 3 Months Ended 0 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 05, 2014
Dec. 31, 2014
Numerators [Abstract]        
Numerator for both basic and diluted EPS, net income $ 77,183,441us-gaap_NetIncomeLoss $ 68,566,071us-gaap_NetIncomeLoss    
Denominators [Abstract]        
Denominator for basic EPS, weighted-average shares outstanding 295,031,431us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 289,836,812us-gaap_WeightedAverageNumberOfSharesOutstandingBasic    
Potentially dilutive shares resulting from stock plans 3,562,712us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment 3,600,202us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment    
Denominator for diluted EPS 298,594,143us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 293,437,014us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding    
Shares related to stock plans not included in diluted average common shares outstanding because their effect would be anti-dilutive 1,184,635us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount 1,188,830us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount    
Common Stock        
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Stock split, conversion ratio     2us-gaap_StockholdersEquityNoteStockSplitConversionRatio1
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonStockMember		 2us-gaap_StockholdersEquityNoteStockSplitConversionRatio1
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonStockMember
Stock split, stock dividend     100.00%gntx_StockholdersEquityNoteStockSplitStockDividendPercent
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonStockMember		 100.00%gntx_StockholdersEquityNoteStockSplitStockDividendPercent
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonStockMember