SELECTED QUARTERLY OPERATING RESULTS (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
Selected Quarterly Financial Information [Abstract]
Revenue	$ 274,207	$ 274,727	$ 284,193	$ 283,143	$ 278,645	$ 271,882	$ 265,391	$ 221,778	$ 1,116,270	$ 1,037,696	$ 590,123
Cost of sales	157,144	152,973	163,091	154,461	165,535	156,492	150,571	122,445	627,669	595,043	276,018
Gross profit	117,063	121,754	121,102	128,682	113,110	115,390	114,820	99,333	488,601	442,653	314,105
Operating income (loss)	46,068	57,742	51,663	61,432	(17,623)	52,240	51,714	24,165	216,905	110,496	160,118
Net income (loss)	$ 36,855	$ 34,525	$ 32,899	$ 38,549	$ (20,243)	$ 18,878	$ 27,137	$ 13,443	$ 142,828	$ 39,215	$ 110,043
Basic earnings (loss) 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 (loss) 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