Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Net sales	$ 429,663	$ 436,671	$ 867,188	$ 873,875
Costs and expenses:
Cost of sales	(380,733)	(361,851)	(767,763)	(730,498)
Selling, general and administrative expenses	(29,265)	(34,655)	(59,202)	(65,450)
Total operating costs and expenses	(409,998)	(396,506)	(826,965)	(795,948)
Income from operations	19,665	40,165	40,223	77,927
Interest expense, net	(7,673)	(7,396)	(15,716)	(15,039)
Earnings (loss) before income taxes	11,992	32,769	24,507	62,888
Income tax provision	(3,955)	(11,905)	(8,955)	(23,578)
Net earnings (loss)	$ 8,037	$ 20,864	$ 15,552	$ 39,310
Net earnings per common share:
Basic (in dollars per share)	$ 0.49	$ 1.22	$ 0.94	$ 2.27
Diluted (in dollars per share)	$ 0.48	$ 1.21	$ 0.94	$ 2.26