Condensed Financial Statements of Greene County Bancorp, Inc., Condensed Statements of Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
OPERATING EXPENSES [Abstract]
Other	$ 2,209	$ 2,038
Net income	14,408	11,187
Parent Company [Member]
INCOME [Abstract]
Equity in undistributed net income of subsidiaries	12,812	9,484
Dividend distributed by subsidiary	1,914	1,904
Interest-earning deposits	2	6
Total Income	14,728	11,394
OPERATING EXPENSES [Abstract]
Legal fees	56	46
Other	264	161
Total operating expenses	320	207
Net income	$ 14,408	$ 11,187