Investments (Maturities Of Fixed Income Securities) (Details) (USD $)	Mar. 31, 2012
Investments [Abstract]
Due within one year	$ 51,989,936
Due between one and five years	3,171,090
Due over five years
Fixed income securities, total	$ 55,161,026

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