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Stock Split (Details) (Common Stock)
0 Months Ended 12 Months Ended
Dec. 05, 2014
Dec. 31, 2014
Dec. 17, 2014
Common Stock
     
Class of Stock [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
 
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)   295,200,000us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonStockMember
147,600,000us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonStockMember