Investments (Maturities of Fixed Income Securities) (Details) (USD $)	Jun. 30, 2013
Investments [Abstract]
Due within one year	$ 92,095,839
Due between one and five years	0
Total	$ 92,095,839

Fixed income securities, maturities due between one and five years.

No definition available.

Name:	gntx_FixedIncomeSecuritiesMaturitiesDueBetweenOneAndFiveYears
Namespace Prefix:	gntx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant