CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 68,341	$ 67,218
Trade receivables, net	22,710	19,602
Inventories, net	12,821	12,846
Prepaid expenses and other current assets	1,476	1,692
Total current assets	105,348	101,358
Property, plant and equipment	353,580	348,472
Less accumulated depreciation and depletion	(208,393)	(205,708)
Property, plant and equipment, net	145,187	142,764
Operating lease right-of-use assets	4,020
Other assets, net	514	549
Total assets	255,069	244,671
Current liabilities:
Accounts payable	6,109	4,570
Current portion of operating lease liabilities	1,425
Accrued expenses	2,343	3,393
Total current liabilities	9,877	7,963
Deferred tax liabilities, net	13,601	12,365
Operating lease liabilities, excluding current portion	2,644
Other liabilities	1,372	1,376
Total liabilities	27,494	21,704
Stockholders' equity:
Common stock	661	661
Additional paid-in capital	26,176	25,867
Accumulated other comprehensive loss	(33)	(13)
Retained earnings	254,939	250,568
Less treasury stock, at cost	(54,168)	(54,116)
Total stockholders' equity	227,575	222,967
Total liabilities and stockholders' equity	$ 255,069	$ 244,671