Consolidated Statements of Income - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Net sales		$ 376,089	$ 347,448	$ 318,263
Cost of sales	[1]	292,490	271,432	252,980
Gross profit		83,599	76,016	65,283
Selling, general, and administrative expenses		33,451	33,107	26,693
Other operating income, net		0	0	(3,385)
Operating income		50,148	42,909	41,975
Gain on sale of equity method investment (Note 4)		0	11,381	0
Equity method investment earnings (loss) (Note 4)		0	(348)	4,036
Interest expense, net		(1,168)	(1,184)	(1,294)
Income before income taxes		48,980	52,758	44,717
Income tax expense (Note 6)		11,696	10,935	13,533
Net income		37,284	41,823	31,184
Income attributable to participating securities		708	996	954
Net income attributable to common shareholders and used in Earnings Per Share calculation (Note 7)		$ 36,576	$ 40,827	$ 30,230
Share information
Basic and diluted weighted average common shares (in shares)		16,866,176	16,746,731	16,643,811
Basic and diluted EPS (in dollars per share)		$ 2.17	$ 2.44	$ 1.82
Dividends and dividend equivalents per common share (in dollars per share)		$ 0.32	$ 1.01	$ 0.12

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