Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 245,679	$ 273,387
Short term investments	162,530	155,846
Trade accounts receivable	183,318	172,604
Other receivables	26,972	29,771
Inventories	163,878	128,960
Other current assets	95,040	70,426
Total current assets	877,417	830,994
Property, plant and equipment, net	919,613	955,948
Other assets
Equity investments	105,325	106,107
Other noncurrent assets	95,267	86,583
Total other assets	200,592	192,690
Total assets	1,997,622	1,979,632
Current Liabilities
Accounts payable	119,629	134,137
Accrued expenses and other current liabilities	203,667	184,161
Current maturities of debt	11,478	15,783
Total current liabilities	334,774	334,081
Long-term debt	302,830	310,134
Asset retirement obligations	319,601	308,855
Accrued pension benefits	5,101	14,036
Accrued postretirement benefits other than pension	105,400	102,369
Accrued workers’ compensation	180,880	184,835
Other noncurrent liabilities	61,896	59,457
Total liabilities	1,310,482	1,313,767
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 25,047 shares at September 30, 2018 and December 31, 2017, respectively	250	250
Paid-in capital	712,295	700,125
Retained earnings	449,122	247,232
Treasury stock, 6,215 shares and 3,977 shares at September 30, 2018 and December 31, 2017, respectively, at cost	(495,232)	(302,109)
Accumulated other comprehensive income	20,705	20,367
Total stockholders’ equity	687,140	665,865
Total liabilities and stockholders’ equity	$ 1,997,622	$ 1,979,632