CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 71,310	$ 54,260
Trade receivables, net	24,846	22,948
Inventories, net	15,062	13,388
Prepaid expenses and other current assets	1,850	2,139
Total current assets	113,068	92,735
Property, plant and equipment	387,577	370,355
Less accumulated depreciation and depletion	(231,534)	(219,668)
Property, plant and equipment, net	156,043	150,687
Operating lease right-of-use assets	2,252	3,192
Other assets, net	344	423
Total assets	271,707	247,037
Current liabilities:
Accounts payable	5,267	4,430
Current portion of operating lease liabilities	1,130	1,294
Accrued expenses	4,675	3,735
Total current liabilities	11,072	9,459
Deferred tax liabilities, net	21,587	17,218
Operating lease liabilities, excluding current portion	1,109	1,866
Other liabilities	1,395	1,362
Total liabilities	35,163	29,905
Stockholders' equity:
Common stock	664	663
Additional paid-in capital	28,888	27,464
Accumulated other comprehensive loss		(1)
Retained earnings	261,834	243,566
Less treasury stock, at cost	(54,842)	(54,560)
Total stockholders' equity	236,544	217,132
Total liabilities and stockholders' equity	$ 271,707	$ 247,037