EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Numerator [Abstract]
Net income	$ 36,855	$ 34,525	$ 32,899	$ 38,549	$ (20,243)	$ 18,878	$ 27,137	$ 13,443	$ 142,828	$ 39,215	$ 110,043
Less: income attributable to participating securities									0	0	(123)
Net income available to common stockholders									$ 142,828	$ 39,215	$ 109,920
Denominator:
Weighted average common shares (in shares)									29,136,000	28,571,000	25,518,000
Weighted average effect of dilutive securities
Weighted average effect of dilutive securities (in shares)									444,000	523,000	725,000
Diluted weighted average common shares (in shares)									29,580,000	29,094,000	26,243,000
Earnings per share:
Basic earnings per share (in dollars per share)	$ 1.27	$ 1.19	$ 1.12	$ 1.32	$ (0.70)	$ 0.65	$ 0.94	$ 0.50	$ 4.90	$ 1.37	$ 4.31
Diluted earnings per share (in dollars per share)	$ 1.25	$ 1.17	$ 1.11	$ 1.30	$ (0.70)	$ 0.64	$ 0.92	$ 0.48	$ 4.83	$ 1.35	$ 4.19
Stock option
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding stock options excluded from calculation of diluted earnings per share (in shares)									102,000	196,000	100,000