Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Current assets
Cash and cash equivalents	$ 243,707	$ 287,807
Short-term investments	35,583	32,724
Restricted cash	1,100	1,100
Trade accounts receivable (net of $0 allowance at June 30, 2024 and December 31, 2023)	241,910	273,522
Other receivables	6,005	13,700
Inventories	249,865	244,261
Other current assets	52,621	64,653
Total current assets	830,791	917,767
Property, plant and equipment, net	1,244,597	1,228,891
Other assets
Deferred income taxes	119,310	124,024
Equity investments	22,861	22,815
Fund for asset retirement obligations	146,010	142,266
Other noncurrent assets	46,999	48,410
Total other assets	335,180	337,515
Total assets	2,410,568	2,484,173
Current Liabilities
Accounts payable	186,549	205,001
Accrued expenses and other current liabilities	111,062	127,617
Current maturities of debt	29,721	35,343
Total current liabilities	327,332	367,961
Long-term debt	101,661	105,252
Asset retirement obligations	263,098	255,740
Accrued pension benefits	832	878
Accrued postretirement benefits other than pension	46,800	47,494
Accrued workers' compensation	157,663	154,650
Other noncurrent liabilities	62,617	72,742
Total liabilities	960,003	1,004,717
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 30,786 and 30,557 shares at June 30, 2024 and December 31, 2023, respectively	308	306
Paid-in capital	758,880	720,029
Retained earnings	1,849,622	1,830,018
Treasury stock, 12,701 and 12,197 shares at June 30, 2024 and December 31, 2023, respectively, at cost	(1,193,876)	(1,109,679)
Accumulated other comprehensive income	35,631	38,782
Total stockholders' equity	1,450,565	1,479,456
Total liabilities and stockholders' equity	$ 2,410,568	$ 2,484,173