Condensed Financial Statements of Greene County Bancorp, Inc., Condensed Statements of Financial Condition (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2014
ASSETS [Abstract]
Total assets	$ 868,781	$ 738,647
LIABILITIES AND SHAREHOLDERS' EQUITY [Abstract]
Total liabilities	794,480	671,727
Total shareholders' equity	74,301	66,920	$ 61,200
Total liabilities and shareholders' equity	868,781	738,647
Parent Company [Member]
ASSETS [Abstract]
Cash and cash equivalents	2,641	544	$ 2,817
Investment in subsidiaries	71,723	66,412
Prepaid expenses and other assets	48	21
Total assets	74,412	66,977
LIABILITIES AND SHAREHOLDERS' EQUITY [Abstract]
Total liabilities	111	57
Total shareholders' equity	74,301	66,920
Total liabilities and shareholders' equity	$ 74,412	$ 66,977