CONSOLIDATED STATEMENTS OF INCOME (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010
CONSOLIDATED STATEMENTS OF INCOME [Abstract]
Revenue	$ 110,621	$ 115,678	$ 99,236	$ 102,122	$ 109,731	$ 111,846	$ 109,660	$ 114,205	$ 427,657	$ 445,442	$ 408,201
Cost of goods sold	56,883	60,462	53,442	52,843	58,814	58,821	56,927	56,774	223,630	231,336	204,704
Gross profit	53,738	55,216	45,794	49,279	50,917	53,025	52,733	57,431	204,027	214,106	203,497
Operating expenses:
Research, development and technical	15,401	15,415	14,071	13,755	14,687	14,573	14,919	13,856	58,642	58,035	51,818
Selling and marketing	7,288	7,458	7,434	7,336	7,702	7,785	6,791	7,480	29,516	29,758	26,885
General and administrative	10,572	10,695	15,177	12,901	11,677	11,008	11,567	11,676	49,345	45,928	50,783
Total operating expenses	33,261	33,568	36,682	33,992	34,066	33,366	33,277	33,012	137,503	133,721	129,486
Operating income	20,477	21,648	9,112	15,287	16,851	19,659	19,456	24,419	66,524	80,385	74,011
Interest expense	961	955	354	39	44	30	37	44	2,309	155	233
Other income (expense), net	(681)	(864)	97	104	(829)	(281)	683	(891)	(1,344)	(1,318)	(501)
Income before income taxes	18,835	19,829	8,855	15,352	15,978	19,348	20,102	23,484	62,871	78,912	73,277
Provision for income taxes	7,196	6,587	3,325	4,937	6,689	6,559	7,010	6,992	22,045	27,250	23,819
Net income	$ 11,639	$ 13,242	$ 5,530	$ 10,415	$ 9,289	$ 12,789	$ 13,092	$ 16,492	$ 40,826	$ 51,662	$ 49,458
Basic earnings per share	$ 0.5100	$ 0.5700	$ 0.2400	$ 0.4600	$ 0.4100	$ 0.5500	$ 0.5700	$ 0.7300	$ 1.8100	$ 2.2600	$ 2.1400
Weighted-average basic shares outstanding	22,920	23,120	22,768	22,508	22,816	23,119	23,032	22,710	22,506	22,896	23,084
Diluted earnings per share	$ 0.4900	$ 0.5500	$ 0.2300	$ 0.4500	$ 0.4000	$ 0.5400	$ 0.5500	$ 0.7100	$ 1.7500	$ 2.2000	$ 2.1300
Weighted-average diluted shares outstanding	23,706	23,939	23,780	22,926	23,191	23,797	23,693	23,131	23,280	23,435	23,273
Dividends per share	$ 0.0000	$ 0.0000	$ 15.0000	$ 0.0000	$ 0.0000	$ 0.0000	$ 0.0000	$ 0.0000	$ 15.0000	$ 0.0000	$ 0.0000