Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 175,428	$ 264,937
Short term investments	176,056	162,797
Trade accounts receivable	206,149	200,904
Other receivables	24,291	48,926
Inventories	171,543	125,470
Other current assets	113,502	75,749
Total current assets	866,969	878,783
Property, plant and equipment, net	889,295	834,828
Other assets
Equity investments	107,543	104,676
Other noncurrent assets	70,793	68,773
Total other assets	178,336	173,449
Total assets	1,934,600	1,887,060
Current Liabilities
Accounts payable	166,129	128,024
Accrued expenses and other current liabilities	161,939	183,514
Current maturities of debt	11,925	17,797
Total current liabilities	339,993	329,335
Long-term debt	292,781	300,186
Asset retirement obligations	238,440	230,304
Accrued pension benefits	12,491	16,147
Accrued postretirement benefits other than pension	78,308	83,163
Accrued workers’ compensation	174,118	174,303
Other noncurrent liabilities	93,033	48,801
Total liabilities	1,229,164	1,182,239
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 25,050 and 25,047 shares at September 30, 2019 and December 31, 2018, respectively	250	250
Paid-in capital	734,829	717,492
Retained earnings	746,928	527,666
Treasury stock, 9,955 shares and 7,216 shares at September 30, 2019 and December 31, 2018, respectively, at cost	(816,882)	(583,883)
Accumulated other comprehensive income	40,311	43,296
Total stockholders’ equity	705,436	704,821
Total liabilities and stockholders’ equity	$ 1,934,600	$ 1,887,060