Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Statement [Abstract]
Revenues		$ 447,506	$ 181,555	$ 801,699	$ 390,435
Costs and expenses:
Cost of goods sold		177,779	149,836	347,081	321,882
Selling, general and administrative expenses		19,512	16,811	36,270	31,018
Total costs and expenses		197,291	166,647	383,351	352,900
Operating income		250,215	14,908	418,348	37,535
Interest expense, net	[1],[2]	(8,199)	(8,339)	(11,773)	(12,037)
Pension settlement charge					(42,988)
Non-operating pension and other postretirement employee benefit costs		(3,271)	(3,478)	(6,685)	(7,113)
Income (loss) before income taxes		238,745	3,091	399,890	(24,603)
Income taxes		(50,840)	(453)	(80,879)	10,409
Net income (loss)		$ 187,905	$ 2,638	$ 319,011	$ (14,194)
Net income (loss) per share:
Basic (in dollars per share)		$ 2.79	$ 0.04	$ 4.74	$ (0.21)
Diluted (in dollars per share)		2.77	0.04	4.71	(0.21)
Dividends per share (in dollars per share)		$ 0.41	$ 0.40	$ 0.82	$ 0.80
Weighted-average shares outstanding:
Basic (in shares)		67,316	67,176	67,265	67,321
Diluted (in shares)		67,732	67,359	67,664	67,321

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

[2] Realized losses on interest rate contracts consist of net cash received or paid and interest accruals on the interest rate swaps during the periods. Net cash received or paid is included in the supplemental cash flow information within interest, net of amounts capitalized in the Condensed Consolidated Statements of Cash Flows.