Investments (Maturities Of Fixed Income Securities) (Details) (USD $)	Jun. 30, 2011
Investments
Due within one year	$ 24,116,803
Due between one and five years	500,000
Due over five years

Name:

gntx_FixedIncomeSecuritiesMaturitiesDueBetweenOneAndFiveYears

Namespace Prefix:

gntx

Data Type:

xbrli:monetaryItemType

Balance Type:

debit

Period Type:

instant