Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Revenues		$ 313,046	$ 226,302	$ 703,481	$ 623,599
Costs and expenses:
Cost of goods sold		182,039	182,634	503,921	512,522
Selling, general and administrative expenses		21,046	12,472	52,064	43,994
Gain on sale of facility					(9,176)
Total costs and expenses		203,085	195,106	555,985	547,340
Operating income		109,961	31,196	147,496	76,259
Interest expense, net	[1]	(8,557)	(8,475)	(20,594)	(21,821)
Loss on extinguishment of debt					(5,512)
Pension settlement charge				(42,988)
Non-operating pension and other postretirement employee benefit costs		(3,557)	(935)	(10,670)	(2,804)
Income before income taxes		97,847	21,786	73,244	46,122
Income taxes		(16,840)	(1,221)	(6,431)	(1,860)
Net income		$ 81,007	$ 20,565	$ 66,813	$ 44,262
Net income per share:
Basic (in dollars per share)		$ 1.21	$ 0.30	$ 0.99	$ 0.65
Diluted (in dollars per share)		1.20	0.30	0.99	0.65
Dividends per share (in dollars per share)		$ 0.40	$ 0.40	$ 1.20	$ 1.20
Weighted-average shares outstanding:
Basic (in shares)		67,149	67,446	67,263	67,781
Diluted (in shares)		67,528	67,545	67,535	67,848

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