CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 73,672	$ 67,218
Trade receivables, net	22,946	19,602
Inventories, net	12,329	12,846
Prepaid expenses and other current assets	1,351	1,692
Total current assets	110,298	101,358
Property, plant and equipment	358,707	348,472
Less accumulated depreciation and depletion	(212,079)	(205,708)
Property, plant and equipment, net	146,628	142,764
Operating lease right-of-use assets	4,054
Other assets, net	493	549
Total assets	261,473	244,671
Current liabilities:
Accounts payable	4,952	4,570
Current portion of operating lease liabilities	1,465
Accrued expenses	3,025	3,393
Total current liabilities	9,442	7,963
Deferred tax liabilities, net	14,974	12,365
Operating lease liabilities, excluding current portion	2,547
Other liabilities	1,369	1,376
Total liabilities	28,332	21,704
Stockholders' equity:
Common stock	662	661
Additional paid-in capital	26,632	25,867
Accumulated other comprehensive loss	(8)	(13)
Retained earnings	260,212	250,568
Less treasury stock, at cost	(54,357)	(54,116)
Total stockholders' equity	233,141	222,967
Total liabilities and stockholders' equity	$ 261,473	$ 244,671