EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Numerator [Abstract]
Net income	$ 48,218	$ 35,171	$ 29,737	$ (3,083)	$ 26,502	$ 19,939	$ 18,280	$ 22,231	$ 110,043	$ 86,952	$ 59,849
Less: income attributable to participating securities									(123)	(256)	(361)
Net income available to common shareholders									$ 109,920	$ 86,696	$ 59,488
Denominator[Abstract]
Weighted-average common shares (Denominator for basic calculation) (in shares)	25,520,000	25,612,000	25,593,000	25,326,000	25,236,000	25,228,000	25,031,000	24,583,000	25,517,825	25,015,458	24,076,549
Weighted-average effect of dilutive securities [Abstract]
Share-based compensation (in shares)									725,339	497,029	400,444
Diluted-weighted average common shares (Denominator for diluted calculation) (in shares)	26,213,000	26,319,000	26,161,000	25,326,000	25,710,000	25,721,000	25,526,000	25,072,000	26,243,164	25,512,487	24,476,993
Earnings per share [Abstract]
Basic (in dollars per share)	$ 1.89	$ 1.37	$ 1.16	$ (0.12)	$ 1.05	$ 0.79	$ 0.73	$ 0.90	$ 4.31	$ 3.47	$ 2.47
Diluted (in dollars per share)	$ 1.84	$ 1.34	$ 1.14	$ (0.12)	$ 1.03	$ 0.77	$ 0.71	$ 0.88	$ 4.19	$ 3.40	$ 2.43
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Abstract]
Outstanding stock options excluded from diluted earnings (in shares)									100,000	400,000	1,100,000