Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Revenues		$ 226,302	$ 289,199	$ 623,599	$ 757,329
Costs and expenses:
Cost of goods sold		182,634	195,584	512,522	515,645
Selling, general and administrative expenses		12,472	14,901	43,994	45,449
Gain on sale of facility				(9,176)
Deltic merger-related costs	[1]		972		21,245
Total costs and expenses		195,106	211,457	547,340	582,339
Operating income		31,196	77,742	76,259	174,990
Interest expense, net	[2]	(8,475)	(10,109)	(21,821)	(25,125)
Loss on extinguishment of debt				(5,512)
Non-operating pension and other postretirement employee benefit costs		(935)	(1,942)	(2,804)	(5,707)
Income before income taxes		21,786	65,691	46,122	144,158
Income tax expense		(1,221)	(5,355)	(1,860)	(23,077)
Net income		$ 20,565	$ 60,336	$ 44,262	$ 121,081
Net income per share:
Basic (in dollars per share)		$ 0.30	$ 0.96	$ 0.65	$ 2.06
Diluted (in dollars per share)		0.30	0.93	0.65	2.03
Dividends per share (in dollars per share)		$ 0.40	$ 0.40	$ 1.20	$ 1.20
Weighted-average shares outstanding:
Basic (in shares)		67,446	62,986	67,781	58,765
Diluted (in shares)		67,545	64,722	67,848	59,542

[1] For integration and restructuring costs related to the merger with Deltic see Note 3: Merger with Deltic.

[2] Bond discounts and deferred loan fees are reported within interest expense, net on the Condensed Consolidated Statements of Income.