Condensed Financial Statements of Greene County Bancorp, Inc., Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Cash flows from operating activities [Abstract]
Net Income	$ 8,963	$ 7,189
Adjustments to reconcile net income to cash provided by (used by) operating activities [Abstract]
Tax benefit of stock based compensation	(6)	(16)
Net decrease (increase) in prepaid expenses and other assets	202	(819)
Net cash provided by operating activities	13,581	12,798
Cash flows from Investing Activities [Abstract]
Net cash used in investing activities	(132,439)	(66,541)
Cash flows from financing activities [Abstract]
Payment of cash dividends	(1,854)	(1,798)
Excess tax benefit from share-based payment arrangements	6	16
Net cash provided by financing activities	119,215	55,472
Net increase (decrease) in cash and cash equivalents	357	1,729
Parent Company [Member]
Cash flows from operating activities [Abstract]
Net Income	8,963	7,189
Adjustments to reconcile net income to cash provided by (used by) operating activities [Abstract]
Undistributed earnings of subsidiaries	(5,239)	(7,491)
Tax benefit of stock based compensation	(6)	(16)
Net decrease (increase) in prepaid expenses and other assets	(27)	(5)
Net increase in total liabilities	61	21
Net cash provided by operating activities	3,752	(302)
Cash flows from Investing Activities [Abstract]
Investment in subsidiary	0	(250)
Net cash used in investing activities	0	(250)
Cash flows from financing activities [Abstract]
Payment of cash dividends	(1,854)	(1,798)
Proceeds from exercise of stock options	193	61
Excess tax benefit from share-based payment arrangements	6	16
Net cash provided by financing activities	(1,655)	(1,721)
Net increase (decrease) in cash and cash equivalents	2,097	(2,273)
Cash and cash equivalents at beginning of year	544	2,817
Cash and cash equivalents at end of year	$ 2,641	$ 544