CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 82,346	$ 85,000
Trade receivables, net	20,923	16,473
Inventories, net	12,257	13,546
Prepaid expenses and other current assets	2,558	2,996
Total current assets	118,084	118,015
Property, plant and equipment	317,902	300,236
Less accumulated depreciation and depletion	(197,909)	(190,518)
Property, plant and equipment, net	119,993	109,718
Other assets, net	605	713
Total assets	238,682	228,446
Current liabilities:
Accounts payable	6,234	6,263
Accrued expenses	2,425	3,096
Total current liabilities	8,659	9,359
Deferred tax liabilities, net	13,504	12,374
Other liabilities	1,432	1,461
Total liabilities	23,595	23,194
Stockholders' equity:
Common stock	660	659
Additional paid-in capital	25,092	24,307
Accumulated other comprehensive (loss) income	(3)	86
Retained earnings	243,294	233,905
Less treasury stock, at cost	(53,956)	(53,705)
Total stockholders' equity	215,087	205,252
Total liabilities and stockholders' equity	$ 238,682	$ 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.