Condensed Financial Statements of Greene County Bancorp, Inc., Condensed Statements of Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Condensed Financial Statements, Captions [Line Items]
Net income	$ 11,187	$ 8,963
Other comprehensive income [Abstract]
Unrealized holding (losses) gains on available-for-sale securities, net of income tax (benefit) expense of ($392) and $328, respectively	(612)	520
Accretion of unrealized loss on securities transferred to held-to-maturity, net of income taxes of $1 and $6, respectively	2	9
Pension actuarial gain (loss), net of income tax expense (benefit) of $158 and ($340)	221	(539)
Amortization of pension actuarial losses recognized in salaries and benefits, net of income taxes of $75 and $52, respectively	122	83
Total other comprehensive (loss) income net of taxes	(267)	73
Comprehensive income	10,920	9,036
Unrealized holding (losses) gains on available-for-sale securities, income tax (benefit) expense	(392)	328
Accretion of unrealized loss on securities transferred to held-to-maturity, income taxes	1	6
Pension actuarial gain (loss), income tax expense (benefit)	158	(340)
Amortization of pension actuarial losses recognized in salaries and benefits, income taxes	75	52
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	11,187	8,963
Other comprehensive income [Abstract]
Unrealized holding (losses) gains on available-for-sale securities, net of income tax (benefit) expense of ($392) and $328, respectively	(612)	520
Accretion of unrealized loss on securities transferred to held-to-maturity, net of income taxes of $1 and $6, respectively	2	9
Pension actuarial gain (loss), net of income tax expense (benefit) of $158 and ($340)	221	(539)
Amortization of pension actuarial losses recognized in salaries and benefits, net of income taxes of $75 and $52, respectively	122	83
Total other comprehensive (loss) income net of taxes	(267)	73
Comprehensive income	10,920	9,036
Unrealized holding (losses) gains on available-for-sale securities, income tax (benefit) expense	(392)	328
Accretion of unrealized loss on securities transferred to held-to-maturity, income taxes	1	6
Pension actuarial gain (loss), income tax expense (benefit)	158	(340)
Amortization of pension actuarial losses recognized in salaries and benefits, income taxes	$ 75	$ 52