Condensed Financial Statements of Greene County Bancorp, Inc., Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities [Abstract]
Net Income	$ 17,484	$ 14,408
Adjustments to reconcile net income to cash provided by operating activities [Abstract]
Net decrease in prepaid expenses and other assets	(43)	(382)
Net cash provided by operating activities	21,625	20,672
Cash flows from Investing Activities [Abstract]
Net cash used in investing activities	(115,339)	(163,378)
Cash flows from financing activities [Abstract]
Payment of cash dividends	(2,037)	(1,526)
Proceeds from exercise of stock options	0	160
Net cash provided by financing activities	96,748	152,933
Net increase in cash and cash equivalents	3,034	10,227
Parent Company [Member]
Cash flows from operating activities [Abstract]
Net Income	17,484	14,408
Adjustments to reconcile net income to cash provided by operating activities [Abstract]
Undistributed earnings of subsidiaries	(14,770)	(12,812)
Net decrease in prepaid expenses and other assets	17	(63)
Net decrease (increase) in total liabilities	6	(6)
Net cash provided by operating activities	2,737	1,527
Cash flows from Investing Activities [Abstract]
Investment in subsidiary	0	(700)
Net cash used in investing activities	0	(700)
Cash flows from financing activities [Abstract]
Payment of cash dividends	(2,037)	(1,526)
Proceeds from exercise of stock options	0	160
Net cash provided by financing activities	(2,037)	(1,366)
Net increase in cash and cash equivalents	700	(539)
Cash and cash equivalents at beginning of year	1,476	2,015
Cash and cash equivalents at end of year	$ 2,176	$ 1,476