Condensed Financial Statements of Greene County Bancorp, Inc., Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities [Abstract]
Net Income	$ 14,408	$ 11,187
Adjustments to reconcile net income to cash provided by operating activities:
Tax benefit of stock based compensation	0	(51)
Net increase in prepaid expenses and other assets	(382)	158
Net cash provided by operating activities	20,672	14,048
Cash flows from Investing Activities [Abstract]
Net cash used in investing activities	(163,378)	(115,764)
Cash flows from financing activities [Abstract]
Payment of cash dividends	(1,526)	(1,920)
Proceeds from exercise of stock options	160	169
Excess tax benefit from share-based payment arrangements	0	51
Net cash provided by financing activities	152,933	102,098
Net increase in cash and cash equivalents	10,227	382
Parent Company [Member]
Cash flows from operating activities [Abstract]
Net Income	14,408	11,187
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries	(12,812)	(9,484)
Tax benefit of stock based compensation	0	(51)
Net increase in prepaid expenses and other assets	(63)	(5)
Net (decrease) increase in total liabilities	(6)	27
Net cash provided by operating activities	1,527	1,674
Cash flows from Investing Activities [Abstract]
Investment in subsidiary	(700)	(600)
Net cash used in investing activities	(700)	(600)
Cash flows from financing activities [Abstract]
Payment of cash dividends	(1,526)	(1,920)
Proceeds from exercise of stock options	160	169
Excess tax benefit from share-based payment arrangements	0	51
Net cash provided by financing activities	(1,366)	(1,700)
Net increase in cash and cash equivalents	(539)	(626)
Cash and cash equivalents at beginning of year	2,015	2,641
Cash and cash equivalents at end of year	$ 1,476	$ 2,015