Summary of Quarterly Results of Operations (Unaudited)) - Quarterly results (Details) - USD ($) $ / shares in Units, $ 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
Summary of Quarterly Results of Operations (Unaudited)
Net sales	$ 379,709	$ 296,212	$ 278,586	$ 248,391	$ 349,115	$ 266,184	$ 248,040	$ 244,370	$ 1,202,898	$ 1,107,709	$ 1,070,601
Operating income (loss)	27,673	10,290	(1,679)	(11,902)	21,207	(6,872)	(6,084)	(5,623)	24,382	2,628	(11,649)
Net income (loss)	$ 17,816	$ 6,291	$ (2,011)	$ (9,786)	$ 13,155	$ (21,957)	$ 461	$ (2,868)	$ 12,310	$ (11,209)	$ 1,735
Net income (loss) per share, basic (in dollars per share)	$ 0.65	$ 0.23	$ (0.07)	$ (0.36)	$ 0.49	$ (0.81)	$ 0.02	$ (0.11)	$ 0.45	$ (0.41)	$ 0.06
Net income (loss) per share, diluted (in dollars per share)	$ 0.65	$ 0.23	$ (0.07)	$ (0.36)	$ 0.49	$ (0.81)	$ 0.02	$ (0.11)	$ 0.45	$ (0.41)	$ 0.06