Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Current assets
Cash and cash equivalents	$ 318,725	$ 325,194
Short-term investments		14,463
Restricted Cash	1,100	1,101
Trade accounts receivable (net of $0 allowance at March 31, 2022 and December 31, 2021)	323,167	324,304
Other receivables	9,807	8,271
Inventories	203,997	156,734
Other current assets	50,217	52,804
Total current assets	907,013	882,871
Property, plant and equipment, net	1,111,359	1,120,043
Other assets
Equity investments	16,494	15,403
Fund for asset retirement obligations	40,000	20,000
Other noncurrent assets	76,019	78,843
Total other assets	132,513	114,246
Total assets	2,150,885	2,117,160
Current Liabilities
Accounts payable	147,284	131,986
Accrued expenses and other current liabilities	179,200	167,304
Current maturities of debt	182,611	223,050
Total current liabilities	509,095	522,340
Long-term debt	132,290	337,623
Asset retirement obligations	193,745	192,672
Accrued pension benefits	948	1,300
Accrued postretirement benefits other than pension	73,828	73,565
Accrued workers' compensation	222,462	224,105
Other noncurrent liabilities	94,265	81,689
Total liabilities	1,226,633	1,433,294
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 25,565 and 25,481 shares at March 31, 2022 and December 31, 2021, respectively	256	255
Paid-in capital	748,999	784,356
Retained earnings	986,797	712,478
Treasury stock, 10,088 shares at March 31, 2022 and December 31, 2021, respectively, at cost	(827,381)	(827,381)
Accumulated other comprehensive income	15,581	14,158
Total stockholders' equity	924,252	683,866
Total liabilities and stockholders' equity	$ 2,150,885	$ 2,117,160