Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information (unaudited)
Net sales	$ 151,825	$ 124,832	$ 163,446	$ 83,964	$ 137,969	$ 125,339	$ 139,371	$ 72,248	$ 524,067	$ 474,927	$ 416,268
Gross profit	44,094	34,920	55,849	20,027	44,811	36,055	45,033	17,187	154,890	143,086	133,974
Income before taxes	19,748	11,069	28,080	(3,138)	21,451	15,081	22,354	(5,971)	55,759	52,915	63,696
Net income	$ 14,696	$ 9,921	$ 21,164	$ (1,876)	$ 34,528	$ 9,327	$ 14,746	$ (3,277)	$ 43,321	$ 54,609	$ 38,469
Basic earnings (loss) per common share (in dollars per share)	$ 0.64	$ 0.43	$ 0.92	$ (0.08)	$ 1.51	$ 0.41	$ 0.64	$ (0.14)	$ 1.91	$ 2.42	$ 1.71
Earnings (loss) per common share assuming dilution (in dollars per share)	0.63	0.43	0.91	(0.08)	1.50	0.40	0.64	(0.14)	$ 1.89	$ 2.40	$ 1.70
Cash dividends paid per share (in dollars per share)	$ 0.27	$ 0.27	$ 0.27	$ 0.27	$ 0.24	$ 0.24	$ 0.24	$ 0.24
Gain (Loss) Related to Litigation Settlement										$ 1,275	$ 10,050