Quarterly Data (unaudited) (Details) (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
Quarterly Data (unaudited)
Net revenues	$ 150,254	$ 141,574	$ 158,882	$ 128,851	$ 154,309	$ 143,122	$ 139,114	$ 106,351	$ 579,561	$ 542,896	$ 381,543
Gross profit	30,425	32,389	34,535	23,491	29,997	25,321	20,593	17,869	120,840	93,780	53,831
Net income	$ 12,856	$ 13,732	$ 15,151	$ 8,443	$ 11,259	$ 8,397	$ 6,216	$ 5,256	$ 49,728	$ 30,808	$ 8,875
Net income per share:
Basic (in dollars per share)	$ 1.05	$ 1.12	$ 1.24	$ 0.69	$ 0.92	$ 0.70	$ 0.52	$ 0.44	$ 4.09	$ 2.55	$ 0.74
Diluted (in dollars per share)	$ 1.04	$ 1.11	$ 1.23	$ 0.68	$ 0.92	$ 0.69	$ 0.51	$ 0.43	$ 4.07	$ 2.54	$ 0.73