Condensed Financial Statements of Greene County Bancorp, Inc., Condensed Statements of Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Condensed Statement of Comprehensive Income [Abstract]
Net income	$ 14,408	$ 11,187
Other comprehensive loss:
Unrealized holding losses on available-for-sale securities, net of income tax benefit of ($319) and ($392), respectively	(659)	(612)
Accretion of unrealized loss on securities transferred to held-to-maturity, net of income tax expense of $- and $1, respectively	0	2
Pension actuarial gain, net of income tax expense of $58 and $158, respectively	163	221
Amortization of pension actuarial losses recognized in salaries and benefits, net of income taxes of $44 and $75, respectively	124	122
Total other comprehensive loss net of taxes	(372)	(267)
Comprehensive income	14,036	10,920
Unrealized holding losses on available-for-sale securities, income taxes	(319)	(392)
Accretion of unrealized loss on securities transferred to held-to-maturity, income taxes	0	1
Pension actuarial gain (loss), income tax expense (benefit)	58	158
Amortization of pension actuarial losses recognized in salaries and benefits, income taxes	44	75
Parent Company [Member]
Condensed Statement of Comprehensive Income [Abstract]
Net income	14,408	11,187
Other comprehensive loss:
Unrealized holding losses on available-for-sale securities, net of income tax benefit of ($319) and ($392), respectively	(659)	(612)
Accretion of unrealized loss on securities transferred to held-to-maturity, net of income tax expense of $- and $1, respectively	0	2
Pension actuarial gain, net of income tax expense of $58 and $158, respectively	163	221
Amortization of pension actuarial losses recognized in salaries and benefits, net of income taxes of $44 and $75, respectively	124	122
Total other comprehensive loss net of taxes	(372)	(267)
Comprehensive income	14,036	10,920
Unrealized holding losses on available-for-sale securities, income taxes	(319)	(392)
Accretion of unrealized loss on securities transferred to held-to-maturity, income taxes	0	1
Pension actuarial gain (loss), income tax expense (benefit)	58	158
Amortization of pension actuarial losses recognized in salaries and benefits, income taxes	$ 44	$ 75