CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 62,958	$ 59,926
Trade receivables, net	17,405	15,889
Inventories, net	13,490	14,728
Prepaid expenses and other current assets	869	1,418
Total current assets	94,722	91,961
Property, plant and equipment	277,463	271,686
Less accumulated depreciation and depletion	(174,136)	(167,308)
Property, plant and equipment, net	103,327	104,378
Other assets, net	151	160
Total assets	198,200	196,499
Current liabilities:
Accounts payable	3,916	6,022
Accrued expenses	2,305	2,720
Total current liabilities	6,221	8,742
Deferred tax liabilities, net	19,280	19,184
Other liabilities	1,889	1,946
Total liabilities	27,390	29,872
Stockholders' equity:
Common stock	656	655
Additional paid-in capital	22,280	21,642
Retained earnings	201,160	194,798
Less treasury stock, at cost	(53,286)	(50,468)
Total stockholders' equity	170,810	166,627
Total liabilities and stockholders' equity	$ 198,200	$ 196,499