SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2015
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 28,949	$ 28,861	$ 27,718	$ 22,575	$ 21,729	$ 20,763	$ 20,629	$ 18,275	$ 16,363	$ 15,254	$ 15,758	$ 14,232
Interest expense	4,461	3,918	3,752	2,742	1,836	844	691	720	660	593	473	661	$ 14,873		$ 4,091		$ 2,387
Net interest income	24,488	24,943	23,966	19,833	19,893	19,919	19,938	17,555	15,703	14,661	15,285	13,571	93,230		77,305		59,220
Provision for loan losses	(144)	1,240	8,649	843	548	2,098	1,173	786	124	700	593	48	10,589		4,605		1,465
Net Income	$ 1,744	$ 9,787	$ 32,142	$ 1,244	$ 6,006	$ 8,873	$ 14,283	$ 4,058	$ 4,639	$ 4,640	$ 5,181	$ 3,595	$ 44,917		$ 33,220		$ 18,055
Earnings (loss) per common and common equivalent share [Abstract]
Basic (in dollars per share)	$ 0.19	$ 1.05	$ 3.44	$ 0.14	$ 0.70	$ 1.04	$ 1.68	$ 0.49	$ 0.64	$ 0.67	$ 0.79	$ 0.58	$ 4.86	[1]	$ 3.93	[1]	$ 2.68	[1]
Diluted (in dollars per share)	0.19	1.04	3.42	0.14	0.70	1.04	1.67	0.49	0.63	0.66	0.78	0.58	$ 4.83	[1]	$ 3.91	[1]	$ 2.66	[1]
Dividend declared per share (in dollars per share)	$ 0.13	$ 0.13	$ 0.13	$ 0.13	$ 0.13	$ 0.13	$ 0.13	$ 0.13	$ 0.13	$ 0.13	$ 0.13	$ 0.13

[1] See Reclassification and Revision of Prior Period Balances under Note 1 Summary of Significant Accounting Policies for additional information describing adjustments made to the Company's EPS calculation. Basic EPS for the fiscal year ended September 30, 2016 of $3.95 was corrected to $3.93 and diluted EPS of $3.92 was corrected to $3.91.