Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended					3 Months Ended								12 Months Ended
	Mar. 14, 2018	May 08, 2017	Aug. 30, 2017	May 25, 2017	Nov. 07, 2016	Sep. 02, 2016	Jan. 29, 2016	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
Net sales	$ 8,000	$ 45,000	$ 14,000	$ 25,000	$ 40,000	$ 25,000	$ 35,000	$ 231,086	$ 192,223	$ 174,778	$ 173,495	$ 207,005	$ 171,189	$ 161,178	$ 146,368	$ 771,582	$ 685,740	$ 571,459
Net earnings								$ 28,452	$ 19,019	$ 9,994	$ 34,671	$ 19,174	$ 12,645	$ 11,157	$ 10,727	$ 92,136	$ 53,703	$ 45,882
Earnings per share:
Basic								$ 1.10	$ 0.73	$ 0.39	$ 1.34	$ 0.74	$ 0.49	$ 0.43	$ 0.42	$ 3.56	$ 2.08	$ 1.78
Diluted								1.09	0.73	0.38	1.33	0.74	0.49	0.43	0.41	3.54	2.07	1.77
Dividends declared per common share								0.08	0.08	0.08	0.08	0.08	0.08	0.08	0.08	$ 0.32	$ 0.32	$ 0.32
Common Stock per share:
High								70.20	60.25	66.80	65.95	63.80	61.40	58.95	58.75
Low								$ 57.00	$ 54.35	$ 57.15	$ 51.55	$ 50.30	$ 55.15	$ 51.80	$ 42.95