Condensed Financial Statements of Greene County Bancorp, Inc., Condensed Statements of Financial Condition (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2019
ASSETS [Abstract]
Total assets	$ 2,200,335	$ 1,676,803
LIABILITIES AND SHAREHOLDERS' EQUITY [Abstract]
Borrowings from other banks, short-term	3,000	17,884
Subordinated notes payable, net	19,644	0
Accrued expenses and other liabilities	22,999	21,439
Total liabilities	2,050,751	1,547,998
Total shareholders' equity	149,584	128,805	$ 112,369
Total liabilities and shareholders' equity	2,200,335	1,676,803
Parent Company [Member]
ASSETS [Abstract]
Cash and cash equivalents	279	2,205	$ 2,176
Investment in subsidiaries	170,705	133,577
Dividend receivable	1,500	0
Prepaid expenses and other assets	108	112
Total assets	172,592	135,894
LIABILITIES AND SHAREHOLDERS' EQUITY [Abstract]
Borrowings from other banks, short-term	3,000	7,000
Subordinated notes payable, net	19,644	0
Accrued expenses and other liabilities	364	89
Total liabilities	23,008	7,089
Total shareholders' equity	149,584	128,805
Total liabilities and shareholders' equity	$ 172,592	$ 135,894