Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets
Cash and cash equivalents	$ 189,707	$ 187,492
Short-term investments	20,084	96,765
Restricted Cash	1,101	5,953
Trade accounts receivable (net of $0 allowance at September 30, 2021 and December 31, 2020)	226,206	110,869
Other receivables	3,573	3,053
Inventories	155,870	126,008
Other current assets	52,098	58,000
Total current assets	648,639	588,140
Property, plant and equipment
Property, plant and equipment, net	1,135,399	1,007,303
Other assets
Equity investments	80,016	71,783
Other noncurrent assets	66,895	55,246
Total other assets	146,911	127,029
Total assets	1,930,949	1,722,472
Current Liabilities
Accounts payable	102,025	103,743
Accrued expenses and other current liabilities	198,268	155,256
Current maturities of debt	138,587	31,097
Total current liabilities	438,880	290,096
Long-term debt	416,446	477,215
Asset retirement obligations	210,843	230,732
Accrued pension benefits	1,527	2,879
Accrued postretirement benefits other than pension	93,317	94,388
Accrued workers' compensation	249,594	244,695
Other noncurrent liabilities	105,699	98,906
Total liabilities	1,516,306	1,438,911
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 25,394 and 25,323 shares at September 30, 2021 and December 31, 2020, respectively	254	253
Paid-in capital	779,013	767,484
Retained earnings	489,914	378,906
Treasury stock, 10,088 shares at September 30, 2021 and December 31, 2020, respectively, at cost	(827,381)	(827,381)
Accumulated other comprehensive loss	(27,157)	(35,701)
Total stockholders' equity	414,643	283,561
Total liabilities and stockholders' equity	$ 1,930,949	$ 1,722,472