Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Current assets
Cash and cash equivalents	$ 156,655	$ 153,020
Short term investments	63,128	135,667
Restricted cash	13,919
Trade accounts receivable (net of $1,303 allowance at September 30, 2020)	127,292	168,125
Other receivables	2,880	21,143
Inventories	143,396	130,898
Other current assets	55,712	97,894
Total current assets	562,982	706,747
Property, plant and equipment, net	964,472	984,509
Other assets
Equity investments	70,275	105,588
Other noncurrent assets	55,608	70,912
Total other assets	125,883	176,500
Total assets	1,653,337	1,867,756
Current Liabilities
Accounts payable	99,110	133,060
Accrued expenses and other current liabilities	141,021	157,167
Current maturities of debt	25,987	20,753
Total current liabilities	266,118	310,980
Long-term debt	368,278	290,066
Asset retirement obligations	239,614	242,432
Accrued pension benefits	6,108	5,476
Accrued postretirement benefits other than pension	85,642	80,567
Accrued workers' compensation	219,851	215,599
Other noncurrent liabilities	102,080	82,100
Total liabilities	1,287,691	1,227,220
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 25,235 and 25,220 shares at September 30, 2020 and December 31, 2019, respectively	252	252
Paid-in capital	744,112	730,551
Retained earnings	457,432	731,425
Treasury stock, 10,088 shares at September 30, 2020 and December 31, 2019, respectively, at cost	(827,381)	(827,381)
Accumulated other comprehensive income (loss)	(8,769)	5,689
Total stockholders' equity	365,646	640,536
Total liabilities and stockholders' equity	$ 1,653,337	$ 1,867,756