Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Current assets
Cash and cash equivalents	$ 189,739	$ 236,059
Short-term investments	32,116	36,993
Restricted cash	1,100	1,100
Trade accounts receivable (net of $0 allowance at March 31, 2023 and December 31, 2022)	295,157	236,999
Other receivables	18,560	18,301
Inventories	271,155	223,015
Other current assets	55,089	71,384
Total current assets	862,916	823,851
Property, plant and equipment, net	1,182,351	1,187,028
Other assets
Deferred income taxes	174,561	209,470
Equity investments	19,142	17,267
Fund for asset retirement obligations	137,134	135,993
Other noncurrent assets	56,555	59,499
Total other assets	387,392	422,229
Total assets	2,432,659	2,433,108
Current Liabilities
Accounts payable	176,953	211,848
Accrued expenses and other current liabilities	120,180	157,043
Current maturities of debt	37,405	57,988
Total current liabilities	334,538	426,879
Long-term debt	110,899	116,288
Asset retirement obligations	237,142	235,736
Accrued pension benefits	1,089	1,101
Accrued postretirement benefits other than pension	51,770	49,674
Accrued workers' compensation	153,870	155,756
Other noncurrent liabilities	79,357	82,094
Total liabilities	968,665	1,067,528
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 30,074 and 28,761 shares at March 31, 2023 and December 31, 2022, respectively	301	288
Paid-in capital	703,712	724,660
Retained earnings	1,705,988	1,565,374
Treasury stock, 11,338 and 11,207 shares at March 31, 2023 and December 31, 2022, respectively, at cost	(1,005,165)	(986,171)
Accumulated other comprehensive income	59,158	61,429
Total stockholders' equity	1,463,994	1,365,580
Total liabilities and stockholders' equity	$ 2,432,659	$ 2,433,108