Summarized Data for Each of the Quarters (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Quarterly Financial Information [Line Items]
Sales	$ 68,288	$ 78,148	$ 76,420	$ 77,751	$ 36,949	$ 55,201	$ 56,233	$ 43,281	$ 300,607	$ 191,664	$ 172,970
Gross profit	29,780	33,875	35,561	40,545	13,386	22,960	25,060	20,658	139,761	82,064	74,113
Total income (loss) from operations	19,132	24,991	25,556	31,548	6,194	15,601	16,472	12,598	101,227	50,865	44,405
Other income (expense), net	274	(50)	(532)	174	326	(14)	351	334	(134)	997	214
Net income	$ 13,684	$ 16,991	$ 16,869	$ 22,013	$ 4,266	$ 10,736	$ 11,431	$ 8,685	$ 69,557	$ 35,118	$ 29,711
Basic	$ 1.06	$ 1.32	$ 1.31	$ 1.72	$ 0.33	$ 0.84	$ 0.90	$ 0.68	$ 5.40	$ 2.76	$ 2.39
Diluted	$ 1.05	$ 1.31	$ 1.30	$ 1.70	$ 0.33	$ 0.83	$ 0.89	$ 0.68	$ 5.38	$ 2.74	$ 2.36