Condensed Financial Statements of Greene County Bancorp, Inc., Condensed Statements of Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Condensed Statement of Comprehensive Income [Abstract]
Net income	$ 17,484	$ 14,408
Other comprehensive income (loss) [Abstract]
Unrealized holding gains (losses) on available-for-sale securities, net of income tax expense (benefit) of $331 and ($319), respectively	936	(659)
Pension actuarial (loss) gain, net of income tax (benefit) expense of ($108) and $58, respectively	(344)	163
Amortization of pension actuarial losses recognized in salaries and benefits, net of income taxes of $36 and $44, respectively	139	124
Total other comprehensive income (loss) net of taxes	731	(372)
Comprehensive income	18,215	14,036
Unrealized holding gains (losses) on available-for-sale securities, income taxes	331	(319)
Pension actuarial (loss) gain, income taxes	(108)	58
Amortization of pension actuarial losses recognized in salaries and benefits, income taxes	36	44
Parent Company [Member]
Condensed Statement of Comprehensive Income [Abstract]
Net income	17,484	14,408
Other comprehensive income (loss) [Abstract]
Unrealized holding gains (losses) on available-for-sale securities, net of income tax expense (benefit) of $331 and ($319), respectively	936	(659)
Pension actuarial (loss) gain, net of income tax (benefit) expense of ($108) and $58, respectively	(344)	163
Amortization of pension actuarial losses recognized in salaries and benefits, net of income taxes of $36 and $44, respectively	139	124
Total other comprehensive income (loss) net of taxes	731	(372)
Comprehensive income	18,215	14,036
Unrealized holding gains (losses) on available-for-sale securities, income taxes	331	(319)
Pension actuarial (loss) gain, income taxes	(108)	58
Amortization of pension actuarial losses recognized in salaries and benefits, income taxes	$ 36	$ 44