CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Current assets:
Cash and cash equivalents	$ 100,864	$ 105,355
Trade receivables, net	32,871	26,715
Inventories, net	16,540	15,116
Prepaid expenses and other current assets	2,491	3,244
Total current assets	152,766	150,430
Property, plant and equipment	424,087	413,561
Less accumulated depreciation and depletion	(255,934)	(251,389)
Property, plant and equipment, net	168,153	162,172
Operating lease right-of-use assets	3,386	3,144
Other assets, net	442	450
Total assets	324,747	316,196
Current liabilities:
Accounts payable	5,474	5,433
Current portion of operating lease liabilities	909	899
Accrued expenses	4,554	4,856
Total current liabilities	10,937	11,188
Deferred tax liabilities, net	23,493	23,055
Operating lease liabilities, excluding current portion	2,572	2,311
Other liabilities	1,439	1,436
Total liabilities	38,441	37,990
Stockholders' equity:
Common stock	669	669
Additional paid-in capital	32,425	31,774
Retained earnings	309,146	301,611
Less treasury stock, at cost	(55,934)	(55,848)
Total stockholders' equity	286,306	278,206
Total liabilities and stockholders' equity	$ 324,747	$ 316,196