Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets
Cash and cash equivalents	$ 273,387	$ 305,372
Short term investments	155,846	88,072
Restricted cash		71,050
Trade accounts receivable	172,604	184,483
Other receivables	29,771	19,877
Inventories	128,960	113,462
Other current assets	70,426	96,306
Total current assets	830,994	878,622
Property, plant and equipment
Coal lands and mineral rights	390,920	387,591
Plant and equipment	445,407	418,182
Deferred mine development	267,063	280,323
Property, plant and equipment, gross	1,103,390	1,086,096
Less accumulated depreciation, depletion and amortization	(147,442)	(32,493)
Property, plant and equipment, net	955,948	1,053,603
Other assets
Prepaid royalties	4,280	0
Deferred income taxes	22,520	0
Equity investments	106,107	96,074
Other noncurrent assets	59,783	108,298
Total other assets	192,690	204,372
Total assets	1,979,632	2,136,597
Current liabilities
Accounts payable	134,137	95,953
Accrued expenses and other current liabilities	184,161	205,240
Current maturities of debt	15,783	11,038
Total current liabilities	334,081	312,231
Long-term debt	310,134	351,841
Asset retirement obligations	308,855	337,227
Accrued pension benefits	14,036	38,884
Accrued postretirement benefits other than pension	102,369	101,445
Accrued workers’ compensation	184,835	184,568
Other noncurrent liabilities	59,457	63,824
Total liabilities	1,313,767	1,390,020
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 25,047 and 25,002 shares at December 31, 2017 and 2016, respectively	250	250
Paid-in capital	700,125	688,424
Treasury stock, 3,977 shares at December 31, 2017, at cost	(302,109)	0
Retained earnings	247,232	33,449
Accumulated other comprehensive income (loss)	20,367	24,454
Total stockholders’ equity	665,865	746,577
Total liabilities and stockholders’ equity	$ 1,979,632	$ 2,136,597