Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Revenues		$ 215,581	$ 268,233	$ 397,297	$ 468,130
Costs and expenses:
Cost of goods sold		175,673	180,906	329,888	320,061
Selling, general and administrative expenses		14,952	16,892	31,522	30,548
Gain on sale of facility				(9,176)
Deltic merger-related costs	[1]		1,018		20,273
Total costs and expenses		190,625	198,816	352,234	370,882
Operating income		24,956	69,417	45,063	97,248
Interest expense, net	[2]	(7,882)	(9,356)	(13,346)	(15,016)
Loss on extinguishment of debt				(5,512)
Non-operating pension and other postretirement employee benefit costs		(889)	(1,908)	(1,869)	(3,765)
Income before income taxes		16,185	58,153	24,336	78,467
Income tax benefit (expense)		952	(12,005)	(639)	(17,722)
Net income		$ 17,137	$ 46,148	$ 23,697	$ 60,745
Net income per share:
Basic (in dollars per share)		$ 0.25	$ 0.73	$ 0.35	$ 1.07
Diluted (in dollars per share)		0.25	0.73	0.35	1.06
Dividends per share (in dollars per share)		$ 0.40	$ 0.40	$ 0.80	$ 0.80
Weighted-average shares outstanding:
Basic (in shares)		67,664	62,980	67,774	56,739
Diluted (in shares)		67,713	63,316	67,866	57,128

[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.