Condensed Financial Statements of Greene County Bancorp, Inc., Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Cash flows from operating activities [Abstract]
Net Income	$ 23,942	$ 18,727
Adjustments to reconcile net income to cash provided by operating activities [Abstract]
Amortization of subordinated debt issuance costs	67	0
Net decrease (increase) in prepaid expenses and other assets	(934)	(1,527)
Net cash provided by operating activities	27,753	26,723
Cash flows from Investing Activities [Abstract]
Net cash used in investing activities	(417,137)	(397,319)
Cash flows from financing activities [Abstract]
Net (decrease) increase in short-term advances - other banks	0	(8,000)
Net proceeds from subordinated notes payable	19,577	0
Payment of cash dividends	(2,430)	(2,238)
Stock repurchase	0	(631)
Net cash provided by financing activities	498,696	381,521
Net increase in cash and cash equivalents	109,312	10,925
Parent Company [Member]
Cash flows from operating activities [Abstract]
Net Income	23,942	18,727
Adjustments to reconcile net income to cash provided by operating activities [Abstract]
Undistributed earnings of subsidiaries	(20,362)	(15,817)
Amortization of subordinated debt issuance costs	68	0
Net decrease (increase) in prepaid expenses and other assets	4	(13)
Net increase in dividend receivable	(1,500)	0
Net increase in total liabilities	275	1
Net cash provided by operating activities	2,427	2,898
Cash flows from Investing Activities [Abstract]
Investment in subsidiary	(17,500)	(7,000)
Net cash used in investing activities	(17,500)	(7,000)
Cash flows from financing activities [Abstract]
Net (decrease) increase in short-term advances - other banks	(4,000)	7,000
Net proceeds from subordinated notes payable	19,577	0
Payment of cash dividends	(2,430)	(2,238)
Stock repurchase	0	(631)
Net cash provided by financing activities	13,147	4,131
Net increase in cash and cash equivalents	(1,926)	29
Cash and cash equivalents at beginning of year	2,205	2,176
Cash and cash equivalents at end of year	$ 279	$ 2,205