Investments (Maturities Of Fixed Income Securities) (Details) (USD $)	Mar. 31, 2013
Investments [Abstract]
Due within one year	$ 75,497,408
Due between one and five years	7,302,340
Total	$ 82,799,748

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