CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 147,575	$ 133,384
Trade receivables, net	38,356	33,592
Inventories, net	22,046	19,579
Prepaid expenses and other current assets	2,826	3,435
Total current assets	210,803	189,990
Property, plant and equipment	445,660	441,597
Less accumulated depreciation and depletion	(274,469)	(269,627)
Property, plant and equipment, net	171,191	171,970
Operating lease right-of-use assets	5,307	5,372
Other assets, net	444	440
Total assets	387,745	367,772
Current liabilities:
Accounts payable	8,830	7,725
Current portion of operating lease liabilities	1,199	1,411
Accrued expenses	8,683	6,401
Total current liabilities	18,712	15,537
Deferred tax liabilities, net	25,421	25,582
Operating lease liabilities, excluding current portion	4,303	4,129
Other liabilities	1,427	1,436
Total liabilities	49,863	46,684
Stockholders' equity:
Common stock	672	671
Additional paid-in capital	35,452	34,528
Retained earnings	358,471	342,504
Less treasury stock, at cost	(56,713)	(56,615)
Total stockholders' equity	337,882	321,088
Total liabilities and stockholders' equity	$ 387,745	$ 367,772