Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Current assets
Cash and cash equivalents	$ 219,595	$ 287,807
Short-term investments	36,347	32,724
Restricted cash	1,100	1,100
Trade accounts receivable (net of $0 allowance at September 30, 2024 and December 31, 2023)	239,994	273,522
Other receivables	8,580	13,700
Inventories	235,091	244,261
Other current assets	47,716	64,653
Total current assets	788,423	917,767
Property, plant and equipment, net	1,238,534	1,228,891
Other assets
Deferred income taxes	126,608	124,024
Equity investments	23,088	22,815
Fund for asset retirement obligations	147,932	142,266
Other noncurrent assets	46,293	48,410
Total other assets	343,921	337,515
Total assets	2,370,878	2,484,173
Current Liabilities
Accounts payable	153,189	205,001
Accrued expenses and other current liabilities	112,767	127,617
Current maturities of debt	126,897	35,343
Total current liabilities	392,853	367,961
Long-term debt		105,252
Asset retirement obligations	267,486	255,740
Accrued pension benefits	816	878
Accrued postretirement benefits other than pension	46,530	47,494
Accrued workers' compensation	158,902	154,650
Other noncurrent liabilities	60,337	72,742
Total liabilities	926,924	1,004,717
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 30,805 and 30,557 shares at September 30, 2024 and December 31, 2023, respectively	308	306
Paid-in capital	764,202	720,029
Retained earnings	1,839,125	1,830,018
Treasury stock, 12,701 and 12,197 shares at September 30, 2024 and December 31, 2023, respectively, at cost	(1,193,884)	(1,109,679)
Accumulated other comprehensive income	34,203	38,782
Total stockholders' equity	1,443,954	1,479,456
Total liabilities and stockholders' equity	$ 2,370,878	$ 2,484,173