Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets
Cash and cash equivalents	$ 187,492	$ 153,020
Short term investments	96,765	135,667
Restricted cash	5,953	0
Trade accounts receivable	110,869	168,125
Other receivables	3,053	21,143
Inventories	126,008	130,898
Other current assets	58,000	97,894
Total current assets	588,140	706,747
Property, plant and equipment
Coal lands and mineral rights	406,095	450,539
Plant and equipment	734,194	639,399
Deferred mine development	288,693	252,108
Property, plant and equipment, gross	1,428,982	1,342,046
Less accumulated depreciation, depletion and amortization	(421,679)	(357,537)
Property, plant and equipment, net	1,007,303	984,509
Other assets
Equity investments	71,783	105,588
Other noncurrent assets	55,246	70,912
Total other assets	127,029	176,500
Total assets	1,722,472	1,867,756
Current Liabilities
Accounts payable	103,743	133,060
Accrued expenses and other current liabilities	155,256	157,167
Current maturities of debt	31,097	20,753
Total current liabilities	290,096	310,980
Long-term debt	477,215	290,066
Asset retirement obligations	230,732	242,432
Accrued pension benefits	2,879	5,476
Accrued postretirement benefits other than pension	94,388	80,567
Accrued workers' compensation	244,695	215,599
Other noncurrent liabilities	98,906	82,100
Total liabilities	1,438,911	1,227,220
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 25,235 and 25,220 shares at December 31, 2020 and December 31, 2019, respectively	253	252
Paid-in capital	767,484	730,551
Retained earnings	378,906	731,425
Treasury stock, 10,088 shares at December 31, 2020 and December 31, 2019, respectively, at cost	(827,381)	(827,381)
Accumulated other comprehensive income (loss)	(35,701)	5,689
Total stockholders' equity	283,561	640,536
Total liabilities and stockholders' equity	$ 1,722,472	$ 1,867,756