Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 232,429	$ 264,937
Short term investments	162,656	162,797
Trade accounts receivable	182,824	200,904
Other receivables	32,950	48,926
Inventories	172,841	125,470
Other current assets	56,993	75,749
Total current assets	840,693	878,783
Property, plant and equipment, net	870,889	834,828
Other assets
Equity investments	106,072	104,676
Other noncurrent assets	71,876	68,773
Total other assets	177,948	173,449
Total assets	1,889,530	1,887,060
Current Liabilities
Accounts payable	138,735	128,024
Accrued expenses and other current liabilities	162,741	183,514
Current maturities of debt	13,068	17,797
Total current liabilities	314,544	329,335
Long-term debt	295,263	300,186
Asset retirement obligations	236,317	230,304
Accrued pension benefits	11,649	16,147
Accrued postretirement benefits other than pension	79,992	83,163
Accrued workers’ compensation	173,621	174,303
Other noncurrent liabilities	80,194	48,801
Total liabilities	1,191,580	1,182,239
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 25,047 shares at June 30, 2019 and December 31, 2018, respectively	250	250
Paid-in capital	728,996	717,492
Retained earnings	647,440	527,666
Treasury stock, 8,785 shares and 7,216 shares at June 30, 2019 and December 31, 2018, respectively, at cost	(725,524)	(583,883)
Accumulated other comprehensive income	46,788	43,296
Total stockholders’ equity	697,950	704,821
Total liabilities and stockholders’ equity	$ 1,889,530	$ 1,887,060