Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 241,590	$ 273,387
Short term investments	160,894	155,846
Trade accounts receivable	198,362	172,604
Other receivables	12,612	29,771
Inventories	157,205	128,960
Other current assets	86,642	70,426
Total current assets	857,305	830,994
Property, plant and equipment, net	925,559	955,948
Other assets
Equity investments	104,189	106,107
Other noncurrent assets	62,360	86,583
Total other assets	166,549	192,690
Total assets	1,949,413	1,979,632
Current Liabilities
Accounts payable	131,027	134,137
Accrued expenses and other current liabilities	194,730	184,161
Current maturities of debt	12,533	15,783
Total current liabilities	338,290	334,081
Long-term debt	305,157	310,134
Asset retirement obligations	316,341	308,855
Accrued pension benefits	7,481	14,036
Accrued postretirement benefits other than pension	106,934	102,369
Accrued workers’ compensation	185,068	184,835
Other noncurrent liabilities	49,194	59,457
Total liabilities	1,308,465	1,313,767
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 25,047 shares at June 30, 2018 and December 31, 2017, respectively	250	250
Paid-in capital	708,127	700,125
Retained earnings	333,753	247,232
Treasury stock, 5,344 shares and 3,977 shares at June 30, 2018 and December 31, 2017, respectively, at cost	(418,985)	(302,109)
Accumulated other comprehensive income	17,803	20,367
Total stockholders’ equity	640,948	665,865
Total liabilities and stockholders’ equity	$ 1,949,413	$ 1,979,632