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Business Combinations (Details) (HomeLink®, USD $)
In Millions, unless otherwise specified
0 Months Ended
Sep. 27, 2013
HomeLink®
 
Business Acquisition [Line Items]  
Purchase price $ 700us-gaap_BusinessCombinationConsiderationTransferred1
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember
Cash paid to acquire business $ 423us-gaap_PaymentsToAcquireBusinessesGross
/ us-gaap_BusinessAcquisitionAxis
= gntx_HomeLinkMember