Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 218,750	$ 264,937
Short term investments	164,664	162,797
Trade accounts receivable	195,095	200,904
Other receivables	44,528	48,926
Inventories	145,607	125,470
Other current assets	64,192	75,749
Total current assets	832,836	878,783
Property, plant and equipment, net	848,749	834,828
Other assets
Equity investments	105,419	104,676
Other noncurrent assets	76,197	68,773
Total other assets	181,616	173,449
Total assets	1,863,201	1,887,060
Current Liabilities
Accounts payable	138,935	128,024
Accrued expenses and other current liabilities	143,586	183,514
Current maturities of debt	15,210	17,797
Total current liabilities	297,731	329,335
Long-term debt	297,733	300,186
Asset retirement obligations	233,614	230,304
Accrued pension benefits	15,452	16,147
Accrued postretirement benefits other than pension	81,296	83,163
Accrued workers’ compensation	168,851	174,303
Other noncurrent liabilities	68,577	48,801
Total liabilities	1,163,254	1,182,239
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 25,047 shares at March 31, 2019 and December 31, 2018, respectively	250	250
Paid-in capital	723,143	717,492
Retained earnings	592,296	527,666
Treasury stock, 8,088 shares and 7,216 shares at March 31, 2019 and December 31, 2018, respectively, at cost	(662,132)	(583,883)
Accumulated other comprehensive income	46,390	43,296
Total stockholders’ equity	699,947	704,821
Total liabilities and stockholders’ equity	$ 1,863,201	$ 1,887,060