CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 83,265	$ 85,000
Trade receivables, net	19,074	16,473
Inventories, net	12,599	13,546
Prepaid expenses and other current assets	2,579	2,996
Total current assets	117,517	118,015
Property, plant and equipment	307,265	300,236
Less accumulated depreciation and depletion	(194,435)	(190,518)
Property, plant and equipment, net	112,830	109,718
Other assets, net	686	713
Total assets	231,033	228,446
Current liabilities:
Accounts payable	5,697	6,263
Accrued expenses	1,999	3,096
Total current liabilities	7,696	9,359
Deferred tax liabilities, net	12,835	12,374
Other liabilities	1,449	1,461
Total liabilities	21,980	23,194
Stockholders' equity:
Common stock	659	659
Additional paid-in capital	24,696	24,307
Accumulated other comprehensive income	52	86
Retained earnings	237,412	233,905
Less treasury stock, at cost	(53,766)	(53,705)
Total stockholders' equity	209,053	205,252
Total liabilities and stockholders' equity	$ 231,033	$ 228,446

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).

Name:	us-gaap_AccountsPayableCurrent
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.