Condensed Financial Statements of Greene County Bancorp, Inc., Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Cash flows from operating activities [Abstract]
Net income	$ 7,189	$ 6,528
Adjustments to reconcile net income to cash provided by (used by) operating activities [Abstract]
Tax benefit of stock based compensation	(16)	(27)
Net increase in prepaid expenses and other assets	(819)	(88)
Net cash provided by operating activities	12,798	11,385
Cash flows from Investing Activities [Abstract]
Net cash used in investing activities	(66,541)	(36,847)
Cash flows from financing activities [Abstract]
Payment of cash dividends	(1,798)	(1,335)
Excess tax benefit from share-based payment arrangements	16	27
Net cash provided by financing activities	55,472	33,049
Net increase in cash and cash equivalents	1,729	7,587
Parent Company [Member]
Cash flows from operating activities [Abstract]
Net income	7,189	6,528
Adjustments to reconcile net income to cash provided by (used by) operating activities [Abstract]
Undistributed earnings of subsidiaries	(7,491)	(4,682)
Tax benefit of stock based compensation	(16)	(28)
Net increase in prepaid expenses and other assets	(5)	0
Net increase in total liabilities	21	20
Net cash provided by operating activities	(302)	1,838
Cash flows from Investing Activities [Abstract]
Investment in subsidiary	(250)	0
Net cash used in investing activities	(250)	0
Cash flows from financing activities [Abstract]
Payment of cash dividends	(1,798)	(1,335)
Proceeds from exercise of stock options	61	172
Excess tax benefit from share-based payment arrangements	16	28
Net cash provided by financing activities	(1,721)	(1,135)
Net increase in cash and cash equivalents	(2,273)	703
Cash and cash equivalents at beginning of year	2,817	2,114
Cash and cash equivalents at end of year	$ 544	$ 2,817