Condensed Financial Statements of Greene County Bancorp, Inc., Condensed Statements of Financial Condition (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2017
ASSETS [Abstract]
Total assets	$ 1,269,462	$ 1,151,478
LIABILITIES AND SHAREHOLDERS' EQUITY [Abstract]
Total liabilities	1,157,093	1,055,287
Total shareholders' equity	112,369	96,191	$ 83,521
Total liabilities and shareholders' equity	1,269,462	1,151,478
Parent Company [Member]
ASSETS [Abstract]
Cash and cash equivalents	2,176	1,476	$ 2,015
Investment in subsidiaries	110,182	94,681
Prepaid expenses and other assets	99	116
Total assets	112,457	96,273
LIABILITIES AND SHAREHOLDERS' EQUITY [Abstract]
Total liabilities	88	82
Total shareholders' equity	112,369	96,191
Total liabilities and shareholders' equity	$ 112,457	$ 96,273