CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 58,796	$ 54,260
Trade receivables, net	23,148	22,948
Inventories, net	13,534	13,388
Prepaid expenses and other current assets	1,890	2,139
Total current assets	97,368	92,735
Property, plant and equipment	375,363	370,355
Less accumulated depreciation and depletion	(224,074)	(219,668)
Property, plant and equipment, net	151,289	150,687
Operating lease right-of-use assets	2,802	3,192
Other assets, net	408	423
Total assets	251,867	247,037
Current liabilities:
Accounts payable	4,831	4,430
Current portion of operating lease liabilities	1,220	1,294
Accrued expenses	2,514	3,735
Total current liabilities	8,565	9,459
Deferred tax liabilities, net	18,198	17,218
Operating lease liabilities, excluding current portion	1,560	1,866
Other liabilities	1,377	1,362
Total liabilities	29,700	29,905
Stockholders' equity:
Common stock	664	663
Additional paid-in capital	27,923	27,464
Accumulated other comprehensive loss	(7)	(1)
Retained earnings	248,211	243,566
Less treasury stock, at cost	(54,624)	(54,560)
Total stockholders' equity	222,167	217,132
Total liabilities and stockholders' equity	$ 251,867	$ 247,037