Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Current assets
Cash and cash equivalents	$ 285,531	$ 287,807
Short-term investments	34,252	32,724
Restricted cash	1,100	1,100
Trade accounts receivable (net of $0 allowance at March 31, 2024 and December 31, 2023)	224,084	273,522
Other receivables	10,519	13,700
Inventories	243,690	244,261
Other current assets	63,393	64,653
Total current assets	862,569	917,767
Property, plant and equipment, net	1,235,775	1,228,891
Other assets
Deferred income taxes	120,792	124,024
Equity investments	24,053	22,815
Fund for asset retirement obligations	144,146	142,266
Other noncurrent assets	47,351	48,410
Total other assets	336,342	337,515
Total assets	2,434,686	2,484,173
Current Liabilities
Accounts payable	178,119	205,001
Accrued expenses and other current liabilities	113,360	127,617
Current maturities of debt	35,341	35,343
Total current liabilities	326,820	367,961
Long-term debt	107,959	105,252
Asset retirement obligations	260,252	255,740
Accrued pension benefits	861	878
Accrued postretirement benefits other than pension	47,384	47,494
Accrued workers' compensation	155,838	154,650
Other noncurrent liabilities	68,124	72,742
Total liabilities	967,238	1,004,717
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 30,785 and 30,557 shares at March 31, 2024 and December 31, 2023, respectively	308	306
Paid-in capital	754,054	720,029
Retained earnings	1,854,621	1,830,018
Treasury stock, 12,607 and 12,197 shares at March 31, 2024 and December 31, 2023, respectively, at cost	(1,178,735)	(1,109,679)
Accumulated other comprehensive income	37,200	38,782
Total stockholders' equity	1,467,448	1,479,456
Total liabilities and stockholders' equity	$ 2,434,686	$ 2,484,173