Quarterly Financial Data (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 Disclosure [Abstract]
Net sales	$ 104,850	$ 95,031	$ 88,252	$ 87,956	$ 88,193	$ 86,333	$ 85,753	$ 87,169	$ 376,089		$ 347,448		$ 318,263
Cost of sales	79,242	75,432	68,811	69,005	68,668	67,708	66,928	68,128	292,490	[1]	271,432	[1]	252,980	[1]
Gross profit	25,608	19,599	19,441	18,951	19,525	18,625	18,825	19,041	83,599		76,016		65,283
SG&A expenses	8,996	7,584	8,309	8,562	8,993	8,154	8,311	7,649	33,451		33,107		26,693
Operating income	16,612	12,015	11,132	10,389	10,532	10,471	10,514	11,392	50,148		42,909		41,975
Gain (loss) on equity method investments					0	11,381	0	0	0		11,381		0
Equity method investment earnings					0	0	(819)	471	0		(348)		4,036
Interest expense, net	(338)	(334)	(289)	(207)	(250)	(224)	(379)	(331)	(1,168)		(1,184)		(1,294)
Income before income taxes	16,274	11,681	10,843	10,182	10,282	21,628	9,316	11,532	48,980		52,758		44,717
Income tax expense (benefit)	4,452	2,673	3,316	1,255	(2,357)	7,491	2,947	2,854	11,696		10,935		13,533
Net income	$ 11,822	$ 9,008	$ 7,527	$ 8,927	$ 12,639	$ 14,137	$ 6,369	$ 8,678	$ 37,284		$ 41,823		$ 31,184
Basic and diluted EPS data (in dollars per share)	$ 0.69	$ 0.52	$ 0.44	$ 0.52	$ 0.74	$ 0.82	$ 0.37	$ 0.50	$ 2.17		$ 2.44		$ 1.82
Dividends and dividend equivalents per common share and per unit (in dollars per share)	$ 0.08	$ 0.08	$ 0.08	$ 0.08	$ 0.04	$ 0.89	$ 0.04	$ 0.04

[1] Includes related party purchases of $0, and $18,425, $29,596 for the years ended December 31, 2018, 2017, and 2016, respectively.