Condensed Financial Statements of Greene County Bancorp, Inc., Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities [Abstract]
Net Income	$ 11,187	$ 8,963
Adjustments to reconcile net income to cash provided by operating activities:
Tax benefit of stock based compensation	(51)	(6)
Net increase in prepaid expenses and other assets	158	202
Net cash provided by operating activities	14,048	13,581
Cash flows from Investing Activities [Abstract]
Net cash used in investing activities	(115,764)	(132,439)
Cash flows from financing activities [Abstract]
Payment of cash dividends	(1,920)	(1,854)
Excess tax benefit from share-based payment arrangements	51	6
Net cash provided by financing activities	102,098	119,215
Net increase in cash and cash equivalents	382	357
Parent Company [Member]
Cash flows from operating activities [Abstract]
Net Income	11,187	8,963
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries	(9,484)	(5,239)
Tax benefit of stock based compensation	(51)	(6)
Net increase in prepaid expenses and other assets	(5)	(27)
Net increase in total liabilities	27	61
Net cash provided by operating activities	1,674	3,752
Cash flows from Investing Activities [Abstract]
Investment in subsidiary	(600)	0
Net cash used in investing activities	(600)	0
Cash flows from financing activities [Abstract]
Payment of cash dividends	(1,920)	(1,854)
Proceeds from exercise of stock options	169	193
Excess tax benefit from share-based payment arrangements	51	6
Net cash provided by financing activities	(1,700)	(1,655)
Net increase in cash and cash equivalents	(626)	2,097
Cash and cash equivalents at beginning of year	2,641	544
Cash and cash equivalents at end of year	$ 2,015	$ 2,641