Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Net sales	$ 489,163	$ 461,375	$ 421,682
Costs, expenses and other income:
Cost of goods sold	361,825	336,317	312,840
Selling and administrative	37,753	37,918	34,891
Costs and Expenses, Related Party	0	0	29,946
Total costs and operating expenses	399,578	374,235	377,677
Operating income	89,585	87,140	44,005
Other expense, net	(1,196)	(5,963)	(751)
Interest expense, net	(11,788)	(13,590)	(16,432)
Income before losses from equity method investment and provision for income tax	76,601	67,587	26,822
Losses from equity method investment	(187)	(736)	(750)
Income before provision for income taxes	76,414	66,851	26,072
Provision for income taxes	(16,566)	(22,827)	(9,336)
Net income	$ 59,848	$ 44,024	$ 16,736
Net income per share:
Basic (usd per share)	$ 1.55	$ 1.08	$ 0.39
Diluted (usd per share)	$ 1.54	$ 1.08	$ 0.39
Weighted average shares outstanding:
Basic (shares)	38,636,152	40,605,464	43,172,528
Diluted (shares)	38,774,963	40,662,304	43,218,324