Condensed Financial Statements of Greene County Bancorp, Inc., Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flows from operating activities [Abstract]
Net Income	$ 18,727	$ 17,484
Adjustments to reconcile net income to cash provided by operating activities [Abstract]
Net (increase) decrease in prepaid expenses and other assets	(1,527)	(43)
Net cash provided by operating activities	26,723	21,625
Cash flows from Investing Activities [Abstract]
Net cash used in investing activities	(397,319)	(115,339)
Cash flows from financing activities [Abstract]
Net (decrease) increase in short-term FHLB advances	(8,000)	8,000
Payment of cash dividends	(2,238)	(2,037)
Stock repurchase	(631)	0
Net cash provided by (used in) financing activities	381,521	96,748
Net increase in cash and cash equivalents	10,925	3,034
Parent Company [Member]
Cash flows from operating activities [Abstract]
Net Income	18,727	17,484
Adjustments to reconcile net income to cash provided by operating activities [Abstract]
Undistributed earnings of subsidiaries	(15,817)	(14,770)
Net (increase) decrease in prepaid expenses and other assets	(13)	17
Net increase in total liabilities	1	6
Net cash provided by operating activities	2,898	2,737
Cash flows from Investing Activities [Abstract]
Investment in subsidiary	(7,000)	0
Net cash used in investing activities	(7,000)	0
Cash flows from financing activities [Abstract]
Net (decrease) increase in short-term FHLB advances	7,000	0
Payment of cash dividends	(2,238)	(2,037)
Stock repurchase	(631)	0
Net cash provided by (used in) financing activities	4,131	(2,037)
Net increase in cash and cash equivalents	29	700
Cash and cash equivalents at beginning of year	2,176	1,476
Cash and cash equivalents at end of year	$ 2,205	$ 2,176