Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Net sales	$ 131,234	$ 116,526	$ 387,304	$ 357,771
Costs, expenses and other income:
Cost of goods sold	94,306	87,952	279,185	267,393
Selling and administrative	9,957	8,867	29,826	27,364
Total costs and operating expenses	104,263	96,819	309,011	294,757
Operating income	26,971	19,707	78,293	63,014
Other (expense)/income, net	(29)	146	(256)	(633)
Interest expense, net	(2,549)	(2,988)	(7,963)	(8,966)
Income before losses from equity method investment and provision for income taxes	24,393	16,865	70,074	53,415
Losses from equity method investment	(393)	(204)	(1,148)	(29)
Income before provision for income taxes	24,000	16,661	68,926	53,386
Provision for income taxes	(5,436)	(5,674)	(14,821)	(17,774)
Net income	$ 18,564	$ 10,987	$ 54,105	$ 35,612
Net income per share:
Basic (usd per share)	$ 0.51	$ 0.29	$ 1.46	$ 0.91
Diluted (usd per share)	$ 0.50	$ 0.29	$ 1.46	$ 0.91
Weighted average shares outstanding:
Basic (shares)	36,732,746	38,212,869	37,012,536	38,966,575
Diluted (shares)	36,918,904	38,345,556	37,181,387	39,080,973