CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Sales	$ 233,022	$ 345,215	$ 535,735	$ 660,110
Costs, expenses and other:
Costs of goods sold	198,739	303,128	470,746	570,008
Selling, general and administrative expenses	17,911	20,009	34,651	39,422
Interest expense, net	1,887	1,328	3,846	2,434
Other non-operating expense (income), net	783	124	549	622
Total costs, expenses and other	219,320	324,589	509,792	612,486
Income before taxes	13,702	20,626	25,943	47,624
Income tax expense	2,273	5,280	5,938	12,104
Net income	$ 11,429	$ 15,346	$ 20,005	$ 35,520
Earnings per common share
Basic (in dollars per share)	$ 0.41	$ 0.54	$ 0.72	$ 1.25
Diluted (in dollars per share)	$ 0.41	$ 0.53	$ 0.71	$ 1.21
Weighted average common shares outstanding
Basic (in shares)	28,012,883	28,162,007	27,977,684	28,489,486
Diluted (in shares)	28,073,648	29,136,979	28,062,301	29,460,149