Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Revenues		$ 287,330	$ 313,046	$ 1,089,029	$ 703,481
Costs and expenses:
Cost of goods sold		190,602	182,039	537,683	503,921
Selling, general and administrative expenses		18,512	21,046	54,782	52,064
Net gain on insurance recoveries		(4,394)		(4,394)
Total costs and expenses		204,720	203,085	588,071	555,985
Operating income		82,610	109,961	500,958	147,496
Interest expense, net	[1]	(8,641)	(8,557)	(20,414)	(20,594)
Pension settlement charge					(42,988)
Non-operating pension and other postretirement employee benefit costs		(3,271)	(3,557)	(9,956)	(10,670)
Income (loss) before income taxes		70,698	97,847	470,588	73,244
Income taxes		(5,031)	(16,840)	(85,910)	(6,431)
Net income (loss)		$ 65,667	$ 81,007	$ 384,678	$ 66,813
Net income (loss) per share:
Basic (in dollars per share)		$ 0.98	$ 1.21	$ 5.72	$ 0.99
Diluted (in dollars per share)		0.97	1.20	5.69	0.99
Dividends per share (in dollars per share)		$ 0.41	$ 0.40	$ 1.23	$ 1.20
Weighted-average shares outstanding:
Basic (in shares)		67,315	67,149	67,275	67,263
Diluted (in shares)		67,648	67,528	67,588	67,535

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