Condensed Financial Statements of Greene County Bancorp, Inc., Condensed Statements of Financial Condition (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2016
ASSETS [Abstract]
Total assets	$ 1,151,478	$ 982,291
LIABILITIES AND SHAREHOLDERS' EQUITY [Abstract]
Total liabilities	1,055,287	898,770
Total shareholders' equity	96,191	83,521	$ 74,301
Total liabilities and shareholders' equity	1,151,478	982,291
Parent Company [Member]
ASSETS [Abstract]
Cash and cash equivalents	1,476	2,015	$ 2,641
Investment in subsidiaries	94,681	81,540
Prepaid expenses and other assets	116	53
Total assets	96,273	83,608
LIABILITIES AND SHAREHOLDERS' EQUITY [Abstract]
Total liabilities	82	87
Total shareholders' equity	96,191	83,521
Total liabilities and shareholders' equity	$ 96,273	$ 83,608