Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current assets
Cash and cash equivalents	$ 298,337	$ 305,372
Short term investments	154,629	88,072
Restricted cash	0	71,050
Trade accounts receivable	207,059	184,483
Other receivables	23,473	19,877
Inventories	122,436	113,462
Other current assets	63,685	96,306
Total current assets	869,619	878,622
Property, plant and equipment, net	980,801	1,053,603
Other assets
Equity investments	106,179	96,074
Other noncurrent assets	65,934	108,298
Total other assets	172,113	204,372
Total assets	2,022,533	2,136,597
Current Liabilities
Accounts payable	137,664	95,953
Accrued expenses and other current liabilities	164,939	205,240
Current maturities of debt	8,566	11,038
Total current liabilities	311,169	312,231
Long-term debt	312,604	351,841
Asset retirement obligations	334,479	337,227
Accrued pension benefits	17,884	38,884
Accrued postretirement benefits other than pension	104,733	101,445
Accrued workers’ compensation	192,262	184,568
Other noncurrent liabilities	76,923	63,824
Total liabilities	1,350,054	1,390,020
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 25,038 shares and 25,002 shares at September 30, 2017 and December 31, 2016, respectively	250	250
Paid-in capital	697,324	688,424
Retained earnings	173,676	33,449
Treasury stock, 2,919 shares at September 30, 2017, at cost	(217,728)	0
Accumulated other comprehensive income	18,957	24,454
Total stockholders’ equity	672,479	746,577
Total liabilities and stockholders’ equity	$ 2,022,533	$ 2,136,597