Condensed Financial Statements of Greene County Bancorp, Inc., Condensed Statements of Financial Condition (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2018
ASSETS [Abstract]
Total assets	$ 1,676,803	$ 1,269,462
LIABILITIES AND SHAREHOLDERS' EQUITY [Abstract]
Borrowings from other banks, short-term	17,884	0
Accrued expenses and other liabilities	21,439	14,924
Total liabilities	1,547,998	1,157,093
Total shareholders' equity	128,805	112,369	$ 96,191
Total liabilities and shareholders' equity	1,676,803	1,269,462
Parent Company [Member]
ASSETS [Abstract]
Cash and cash equivalents	2,205	2,176	$ 1,476
Investment in subsidiaries	133,577	110,182
Prepaid expenses and other assets	112	99
Total assets	135,894	112,457
LIABILITIES AND SHAREHOLDERS' EQUITY [Abstract]
Borrowings from other banks, short-term	7,000	0
Accrued expenses and other liabilities	89	88
Total liabilities	7,089	88
Total shareholders' equity	128,805	112,369
Total liabilities and shareholders' equity	$ 135,894	$ 112,457