CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 68,835	$ 54,260
Trade receivables, net	21,192	22,948
Inventories, net	14,422	13,388
Prepaid expenses and other current assets	1,243	2,139
Total current assets	105,692	92,735
Property, plant and equipment	378,988	370,355
Less accumulated depreciation and depletion	(228,328)	(219,668)
Property, plant and equipment, net	150,660	150,687
Operating lease right-of-use assets	2,536	3,192
Other assets, net	382	423
Total assets	259,270	247,037
Current liabilities:
Accounts payable	4,212	4,430
Current portion of operating lease liabilities	1,169	1,294
Accrued expenses	3,843	3,735
Total current liabilities	9,224	9,459
Deferred tax liabilities, net	19,692	17,218
Operating lease liabilities, excluding current portion	1,352	1,866
Other liabilities	1,383	1,362
Total liabilities	31,651	29,905
Stockholders' equity:
Common stock	664	663
Additional paid-in capital	28,386	27,464
Accumulated other comprehensive loss		(1)
Retained earnings	253,411	243,566
Less treasury stock, at cost	(54,842)	(54,560)
Total stockholders' equity	227,619	217,132
Total liabilities and stockholders' equity	$ 259,270	$ 247,037