Investments (Maturities Of Fixed Income Securities) (Details) (USD $)	Sep. 30, 2012
Investments [Abstract]
Due within one year	$ 21,522,299
Due between one and five years	11,326,302
Due over five years
Total	$ 32,848,601

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