Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Current assets
Cash and cash equivalents	$ 201,492	$ 236,059
Short-term investments	33,607	36,993
Restricted cash	1,100	1,100
Trade accounts receivable (net of $0 allowance at June 30, 2023 and December 31, 2022)	250,479	236,999
Other receivables	18,440	18,301
Inventories	263,310	223,015
Other current assets	49,547	71,384
Total current assets	817,975	823,851
Property, plant and equipment, net	1,192,681	1,187,028
Other assets
Deferred income taxes	160,927	209,470
Equity investments	22,348	17,267
Fund for asset retirement obligations	138,657	135,993
Other noncurrent assets	54,313	59,499
Total other assets	376,245	422,229
Total assets	2,386,901	2,433,108
Current Liabilities
Accounts payable	188,494	211,848
Accrued expenses and other current liabilities	121,043	157,043
Current maturities of debt	26,414	57,988
Total current liabilities	335,951	426,879
Long-term debt	109,216	116,288
Asset retirement obligations	234,842	235,736
Accrued pension benefits	1,076	1,101
Accrued postretirement benefits other than pension	50,802	49,674
Accrued workers' compensation	152,942	155,756
Other noncurrent liabilities	76,508	82,094
Total liabilities	961,337	1,067,528
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 30,104 and 28,761 shares at June 30, 2023 and December 31, 2022, respectively	301	288
Paid-in capital	710,118	724,660
Retained earnings	1,737,661	1,565,374
Treasury stock, 11,905 and 11,207 shares at June 30, 2023 and December 31, 2022, respectively, at cost	(1,079,396)	(986,171)
Accumulated other comprehensive income	56,880	61,429
Total stockholders' equity	1,425,564	1,365,580
Total liabilities and stockholders' equity	$ 2,386,901	$ 2,433,108