Quarterly Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Quarterly Data (unaudited)
Net revenues	$ 129,640	$ 126,032	$ 127,474	$ 107,069	$ 122,313	$ 113,114	$ 110,206	$ 89,693	$ 490,215	$ 435,326	$ 395,209
Gross profit	$ 21,310	$ 18,175	$ 14,683	$ 11,335	$ 17,884	$ 15,363	$ 13,513	$ 9,075	65,503	55,835	45,689
Gross profit percentage of net revenue (as a percent)	16.40%	14.40%	11.50%	10.60%	14.60%	13.60%	12.30%	10.10%
Net income (loss)	$ 6,037	$ 3,802	$ 1,509	$ (1,603)	$ 2,130	$ 713	$ (2,068)	$ (22,526)	$ 9,745	$ (21,751)	$ (10,190)
Net income (loss) per share:
Basic (in dollars per share)	$ 0.48	$ 0.30	$ 0.12	$ (0.13)	$ 0.17	$ 0.06	$ (0.17)	$ (1.82)	$ 0.78	$ (1.75)	$ (0.83)
Diluted (in dollars per share)	$ 0.48	$ 0.30	$ 0.12	$ (0.13)	$ 0.17	$ 0.06	$ (0.17)	$ (1.82)	$ 0.78	$ (1.75)	$ (0.83)