Summary of Quarterly Results of Operations (Unaudited)) - Quarterly results (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
Summary of Quarterly Results of Operations (Unaudited)
Net sales	$ 471,198	$ 382,679	$ 337,339	$ 305,259	$ 379,709	$ 296,212	$ 278,586	$ 248,391	$ 1,496,475	$ 1,202,898	$ 1,107,709
Operating income (loss)	58,619	34,725	(6,541)	(566)	27,673	10,290	(1,679)	(11,902)	86,237	24,382	2,628
Net income (loss)	$ 41,763	$ 23,910	$ (9,392)	$ (6,587)	$ 17,816	$ 6,291	$ (2,011)	$ (9,786)	$ 49,694	$ 12,310	$ (11,209)
Net income (loss) per share, basic (in dollars per share)	$ 1.39	$ 0.77	$ (0.30)	$ (0.23)	$ 0.65	$ 0.23	$ (0.07)	$ (0.36)	$ 1.63	$ 0.45	$ (0.41)
Net income (loss) per share, diluted (in dollars per share)	$ 1.38	$ 0.77	$ (0.30)	$ (0.23)	$ 0.65	$ 0.23	$ (0.07)	$ (0.36)	$ 1.62	$ 0.45	$ (0.41)