Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 153,020	$ 264,937
Short-term investments	135,667	162,797
Trade accounts receivable	168,125	200,904
Other receivables	21,143	48,926
Inventories	130,898	125,470
Other current assets	97,894	75,749
Total current assets	706,747	878,783
Property, plant and equipment
Coal lands and mineral rights	450,539	396,125
Plant and equipment	639,399	510,683
Deferred mine development	252,108	194,363
Property, plant and equipment, gross	1,342,046	1,101,171
Less accumulated depreciation, depletion and amortization	(357,537)	(266,343)
Property, plant and equipment, net	984,509	834,828
Other assets
Prepaid royalties	1,600	600
Deferred income taxes	82	170
Equity investments	105,588	104,676
Other noncurrent assets	69,230	68,003
Other Noncurrent Assets, Excluding Property, Plant And Equipment	176,500	173,449
Total assets	1,867,756	1,887,060
Current liabilities
Accounts payable	133,060	128,024
Accrued expenses and other current liabilities	157,167	183,514
Current maturities of debt	20,753	17,797
Total current liabilities	310,980	329,335
Long-term debt	290,066	300,186
Asset retirement obligations	242,432	230,304
Accrued pension benefits	5,476	16,147
Accrued postretirement benefits other than pension	80,567	83,163
Accrued workers’ compensation	215,599	174,303
Other noncurrent liabilities	82,100	48,801
Total liabilities	1,227,220	1,182,239
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 25,220 and 25,047 shares at December 31, 2019 and 2018, respectively	252	250
Paid-in capital	730,551	717,492
Treasury stock, 10,088 and 7,216 shares at December 31, 2019 and 2018, respectively, at cost	(827,381)	(583,883)
Retained earnings	731,425	527,666
Accumulated other comprehensive income	5,689	43,296
Total stockholders’ equity	640,536	704,821
Total liabilities and stockholders’ equity	$ 1,867,756	$ 1,887,060