Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Current assets
Cash and cash equivalents	$ 153,516	$ 187,492
Short-term investments	33,256	96,765
Restricted Cash	7,104	5,953
Trade accounts receivable (net of $0 allowance at June 30, 2021 and December 31, 2020)	160,154	110,869
Other receivables	3,336	3,053
Inventories	162,531	126,008
Other current assets	35,149	58,000
Total current assets	555,046	588,140
Property, plant and equipment
Property, plant and equipment, net	1,102,570	1,007,303
Other assets
Equity investments	74,974	71,783
Other noncurrent assets	62,356	55,246
Total other assets	137,330	127,029
Total assets	1,794,946	1,722,472
Current Liabilities
Accounts payable	116,042	103,743
Accrued expenses and other current liabilities	166,287	155,256
Current maturities of debt	133,812	31,097
Total current liabilities	416,141	290,096
Long-term debt	402,076	477,215
Asset retirement obligations	213,387	230,732
Accrued pension benefits	2,274	2,879
Accrued postretirement benefits other than pension	95,491	94,388
Accrued workers' compensation	249,256	244,695
Other noncurrent liabilities	97,792	98,906
Total liabilities	1,476,417	1,438,911
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 25,382 and 25,323 shares at June 30, 2021 and December 31, 2020, respectively	254	253
Paid-in capital	774,665	767,484
Retained earnings	400,771	378,906
Treasury stock, 10,088 shares at June 30, 2021 and December 31, 2020, respectively, at cost	(827,381)	(827,381)
Accumulated other comprehensive loss	(29,780)	(35,701)
Total stockholders' equity	318,529	283,561
Total liabilities and stockholders' equity	$ 1,794,946	$ 1,722,472