Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Current assets
Cash and cash equivalents	$ 150,022	$ 153,020
Short term investments	67,237	135,667
Trade accounts receivable (net of $718 allowance at June 30, 2020)	118,835	168,125
Other receivables	2,960	21,143
Inventories	154,690	130,898
Other current assets	70,758	97,894
Total current assets	564,502	706,747
Property, plant and equipment, net	1,066,614	984,509
Other assets
Equity investments	106,125	105,588
Other noncurrent assets	65,148	70,912
Total other assets	171,273	176,500
Total assets	1,802,389	1,867,756
Current Liabilities
Accounts payable	104,464	133,060
Accrued expenses and other current liabilities	143,304	157,167
Current maturities of debt	25,702	20,753
Total current liabilities	273,470	310,980
Long-term debt	323,854	290,066
Asset retirement obligations	238,883	242,432
Accrued pension benefits	13,875	5,476
Accrued postretirement benefits other than pension	86,772	80,567
Accrued workers' compensation	219,095	215,599
Other noncurrent liabilities	98,658	82,100
Total liabilities	1,254,607	1,227,220
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 25,235 and 25,220 shares at June 30, 2020 and December 31, 2019, respectively	252	252
Paid-in capital	739,156	730,551
Retained earnings	648,909	731,425
Treasury stock, 10,088 shares at June 30, 2020 and December 31, 2019, respectively, at cost	(827,381)	(827,381)
Accumulated other comprehensive income (loss)	(13,154)	5,689
Total stockholders' equity	547,782	640,536
Total liabilities and stockholders' equity	$ 1,802,389	$ 1,867,756