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Stock Split (Details)
0 Months Ended
Dec. 05, 2014
Dec. 31, 2014
Dec. 31, 2013
Dec. 17, 2014
Class of Stock [Line Items]        
Common stock, shares outstanding (in shares)   295,247,958us-gaap_CommonStockSharesOutstanding 291,156,046us-gaap_CommonStockSharesOutstanding  
Common Stocks        
Class of Stock [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
     
Common stock split, additional shares issued for every share held (in shares)       1gntx_StockholdersEquityNoteStockSplitAdditionalSharesIssuedforEveryShareHeld
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonStockMember
Common stock, shares outstanding (in shares)   295,200,000.0us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonStockMember
  147,600,000.0us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonStockMember