Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Current assets
Cash and cash equivalents	$ 281,944	$ 325,194
Short-term investments		14,463
Restricted cash	1,100	1,101
Trade accounts receivable (net of $0 allowance at June 30, 2022 and December 31, 2021)	457,883	324,304
Other receivables	12,847	8,271
Inventories	212,752	156,734
Other current assets	54,805	52,804
Total current assets	1,021,331	882,871
Property, plant and equipment, net	1,108,926	1,120,043
Other assets
Equity investments	16,786	15,403
Fund for asset retirement obligations	100,000	20,000
Other noncurrent assets	66,604	78,843
Total other assets	183,390	114,246
Total assets	2,313,647	2,117,160
Current Liabilities
Accounts payable	165,143	131,986
Accrued expenses and other current liabilities	191,125	167,304
Current maturities of debt	55,920	223,050
Total current liabilities	412,188	522,340
Long-term debt	127,107	337,623
Asset retirement obligations	194,249	192,672
Accrued pension benefits	591	1,300
Accrued postretirement benefits other than pension	74,300	73,565
Accrued workers' compensation	222,289	224,105
Other noncurrent liabilities	94,215	81,689
Total liabilities	1,124,939	1,433,294
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 28,642 and 25,481 shares at June 30, 2022 and December 31, 2021, respectively	286	255
Paid-in capital	774,144	784,356
Retained earnings	1,234,979	712,478
Treasury stock, 10,088 shares at June 30, 2022 and December 31, 2021, respectively, at cost	(827,381)	(827,381)
Accumulated other comprehensive income	6,680	14,158
Total stockholders' equity	1,188,708	683,866
Total liabilities and stockholders' equity	$ 2,313,647	$ 2,117,160