CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 89,651	$ 83,562
Trade receivables, net	23,957	22,979
Inventories, net	15,409	15,210
Prepaid expenses and other current assets	2,428	2,245
Total current assets	131,445	123,996
Property, plant and equipment	392,631	388,200
Less accumulated depreciation and depletion	(240,993)	(235,739)
Property, plant and equipment, net	151,638	152,461
Operating lease right-of-use assets	1,929	2,226
Other assets, net	398	415
Total assets	285,410	279,098
Current liabilities:
Accounts payable	5,675	4,592
Current portion of operating lease liabilities	1,073	1,187
Accrued expenses	3,995	5,809
Total current liabilities	10,743	11,588
Deferred tax liabilities, net	22,271	21,531
Operating lease liabilities, excluding current portion	858	1,030
Other liabilities	1,768	1,757
Total liabilities	35,640	35,906
Stockholders' equity:
Common stock	667	666
Additional paid-in capital	30,003	29,457
Retained earnings	274,312	268,186
Less treasury stock, at cost	(55,212)	(55,117)
Total stockholders' equity	249,770	243,192
Total liabilities and stockholders' equity	$ 285,410	$ 279,098