Summary of Quarterly Results of Operations (Unaudited)) - Quarterly results (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Summary of Quarterly Results of Operations (Unaudited)
Net sales	$ 475,475	$ 350,439	$ 321,482	$ 328,839	$ 471,198	$ 382,679	$ 337,339	$ 305,259	$ 1,476,235	$ 1,496,475	$ 1,202,898
Operating income (loss)	73,286	24,706	(29,934)	(6,483)	58,619	34,725	(6,541)	(566)	61,575	86,237	24,382
Net income (loss)	$ 57,632	$ (1,155)	$ (39,123)	$ (20,575)	$ 41,763	$ 23,910	$ (9,392)	$ (6,587)	$ (3,221)	$ 49,694	$ 12,310
Net income (loss) per share, basic (in dollars per share)	$ 1.85	$ (0.04)	$ (1.25)	$ (0.66)	$ 1.39	$ 0.77	$ (0.30)	$ (0.23)	$ (0.10)	$ 1.63	$ 0.45
Net income (loss) per share, diluted (in dollars per share)	$ 1.84	$ (0.04)	$ (1.25)	$ (0.66)	$ 1.38	$ 0.77	$ (0.30)	$ (0.23)	$ (0.10)	$ 1.62	$ 0.45