Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Current assets
Cash and cash equivalents	$ 490,321	$ 325,194
Short-term investments	10,671	14,463
Restricted cash	1,100	1,101
Trade accounts receivable (net of $0 allowance at September 30, 2022 and December 31, 2021)	210,349	324,304
Other receivables	13,592	8,271
Inventories	215,172	156,734
Other current assets	49,869	52,804
Total current assets	991,074	882,871
Property, plant and equipment, net	1,116,550	1,120,043
Other assets
Equity investments	17,044	15,403
Fund for asset retirement obligations	130,000	20,000
Other noncurrent assets	65,194	78,843
Total other assets	212,238	114,246
Total assets	2,319,862	2,117,160
Current Liabilities
Accounts payable	186,322	131,986
Accrued expenses and other current liabilities	160,247	167,304
Current maturities of debt	52,179	223,050
Total current liabilities	398,748	522,340
Long-term debt	121,914	337,623
Asset retirement obligations	195,655	192,672
Accrued pension benefits	493	1,300
Accrued postretirement benefits other than pension	72,890	73,565
Accrued workers' compensation	222,648	224,105
Other noncurrent liabilities	121,453	81,689
Total liabilities	1,133,801	1,433,294
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 28,642 and 25,481 shares at September 30, 2022 and December 31, 2021, respectively	286	255
Paid-in capital	766,427	784,356
Retained earnings	1,299,024	712,478
Treasury stock, 10,517 and 10,088 shares at September 30, 2022 and December 31, 2021, respectively, at cost	(884,879)	(827,381)
Accumulated other comprehensive income	5,203	14,158
Total stockholders' equity	1,186,061	683,866
Total liabilities and stockholders' equity	$ 2,319,862	$ 2,117,160