Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Current assets
Cash and cash equivalents	$ 183,097	$ 236,059
Short-term investments	30,447	36,993
Restricted cash	1,100	1,100
Trade accounts receivable (net of $0 allowance at September 30, 2023 and December 31, 2022)	289,994	236,999
Other receivables	16,439	18,301
Inventories	233,213	223,015
Other current assets	51,854	71,384
Total current assets	806,144	823,851
Property, plant and equipment, net	1,200,109	1,187,028
Other assets
Deferred income taxes	146,324	209,470
Equity investments	21,273	17,267
Fund for asset retirement obligations	140,413	135,993
Other noncurrent assets	51,595	59,499
Total other assets	359,605	422,229
Total assets	2,365,858	2,433,108
Current Liabilities
Accounts payable	198,374	211,848
Accrued expenses and other current liabilities	135,163	157,043
Current maturities of debt	21,926	57,988
Total current liabilities	355,463	426,879
Long-term debt	107,515	116,288
Asset retirement obligations	234,386	235,736
Accrued pension benefits	1,057	1,101
Accrued postretirement benefits other than pension	50,360	49,674
Accrued workers' compensation	144,714	155,756
Other noncurrent liabilities	76,213	82,094
Total liabilities	969,708	1,067,528
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 30,501 and 28,761 shares at September 30, 2023 and December 31, 2022, respectively	305	288
Paid-in capital	716,956	724,660
Retained earnings	1,737,468	1,565,374
Treasury stock, 12,168 and 11,207 shares at September 30, 2023 and December 31, 2022, respectively, at cost	(1,107,878)	(986,171)
Accumulated other comprehensive income	49,299	61,429
Total stockholders' equity	1,396,150	1,365,580
Total liabilities and stockholders' equity	$ 2,365,858	$ 2,433,108