CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ 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
CONSOLIDATED STATEMENTS OF INCOME [Abstract]
Revenue	$ 156,729	$ 150,437	$ 142,978	$ 139,979	$ 136,784	$ 127,957	$ 119,184	$ 123,254	$ 590,123	$ 507,179	$ 430,449
Cost of goods sold	72,383	69,737	67,933	65,965	66,734	65,414	59,153	61,749	276,018	253,050	220,247
Gross profit	84,346	80,700	75,045	74,014	70,050	62,543	60,031	61,505	314,105	254,129	210,202
Operating expenses:
Research, development and technical	13,372	13,059	13,368	12,151	13,839	14,333	14,090	13,396	51,950	55,658	58,532
Selling and marketing	6,211	6,207	6,790	5,836	8,680	7,346	7,268	7,552	25,044	30,846	27,717
General and administrative	20,775	19,504	17,799	18,915	14,489	13,953	14,699	12,496	76,993	55,637	49,445
Total operating expenses	40,358	38,770	37,957	36,902	37,008	35,632	36,057	33,444	153,987	142,141	135,694
Operating income	43,988	41,930	37,088	37,112	33,042	26,911	23,974	28,061	160,118	111,988	74,508
Interest expense	102	513	1,158	1,132	1,127	1,117	1,135	1,150	2,905	4,529	4,723
Other income, net	1,137	1,627	1,062	672	798	(115)	234	996	4,498	1,913	653
Income before income taxes	45,023	43,044	36,992	36,652	32,713	25,679	23,073	27,907	161,711	109,372	70,438
Provision for income taxes	(3,195)	7,873	7,255	39,735	6,211	5,740	4,793	5,676	51,668	22,420	10,589
Net income	$ 48,218	$ 35,171	$ 29,737	$ (3,083)	$ 26,502	$ 19,939	$ 18,280	$ 22,231	$ 110,043	$ 86,952	$ 59,849
Basic earnings per share (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
Weighted average basic shares outstanding (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
Diluted earnings per share (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
Weighted average diluted shares outstanding (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
Dividends per share (in dollars per share)	$ 0.40	$ 0.40	$ 0.40	$ 0.20	$ 0.20	$ 0.20	$ 0.20	$ 0.18	$ 1.40	$ 0.78	$ 0.54