Condensed Financial Statements of Greene County Bancorp, Inc., Condensed Statements of Financial Condition (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2015
ASSETS [Abstract]
Total assets	$ 982,291	$ 868,781
LIABILITIES AND SHAREHOLDERS' EQUITY [Abstract]
Total liabilities	898,770	794,480
Total shareholders' equity	83,521	74,301	$ 66,920
Total liabilities and shareholders' equity	982,291	868,781
Parent Company [Member]
ASSETS [Abstract]
Cash and cash equivalents	2,015	2,641	$ 544
Investment in subsidiaries	81,540	71,723
Prepaid expenses and other assets	53	48
Total assets	83,608	74,412
LIABILITIES AND SHAREHOLDERS' EQUITY [Abstract]
Total liabilities	87	111
Total shareholders' equity	83,521	74,301
Total liabilities and shareholders' equity	$ 83,608	$ 74,412