Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 288,332	$ 273,387
Short term investments	144,652	155,846
Restricted cash	2,926	0
Trade accounts receivable	208,100	172,604
Other receivables	11,405	29,771
Inventories	143,831	128,960
Other current assets	68,459	70,426
Total current assets	867,705	830,994
Property, plant and equipment, net	935,511	955,948
Other assets
Equity investments	104,786	106,107
Other noncurrent assets	68,676	86,583
Total other assets	173,462	192,690
Total assets	1,976,678	1,979,632
Current Liabilities
Accounts payable	127,149	134,137
Accrued expenses and other current liabilities	158,882	184,161
Current maturities of debt	14,328	15,783
Total current liabilities	300,359	334,081
Long-term debt	307,742	310,134
Asset retirement obligations	312,544	308,855
Accrued pension benefits	13,370	14,036
Accrued postretirement benefits other than pension	104,902	102,369
Accrued workers’ compensation	185,159	184,835
Other noncurrent liabilities	64,140	59,457
Total liabilities	1,288,216	1,313,767
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 25,047 shares at March 31, 2018 and December 31, 2017, respectively	250	250
Paid-in capital	703,980	700,125
Retained earnings	298,664	247,232
Treasury stock, 4,384 shares and 3,977 shares at March 31, 2018 and December 31, 2017, respectively, at cost	(340,698)	(302,109)
Accumulated other comprehensive income	26,266	20,367
Total stockholders’ equity	688,462	665,865
Total liabilities and stockholders’ equity	$ 1,976,678	$ 1,979,632

Carrying value as of the balance sheet date of liabilities incurred (and for which invoices have typically been received) and payable to vendors for goods and services received that are used in an entity's business. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer), for goods or services (including trade receivables) that have been delivered or sold in the normal course of business, reduced to the estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection.