Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 105,355	$ 83,562
Trade receivables, net	26,715	22,979
Inventories, net	15,116	15,210
Prepaid expenses and other current assets	3,244	2,245
Total current assets	150,430	123,996
Property, plant and equipment:
Mineral reserves and land	40,534	40,065
Proved natural gas properties, successful-efforts method	15,934	15,934
Buildings and building and leasehold improvements	7,856	7,808
Machinery and equipment	342,120	318,503
Furniture and fixtures	1,173	1,088
Automotive equipment	5,944	4,802
Property, plant and equipment	413,561	388,200
Less accumulated depreciation and depletion	(251,389)	(235,739)
Property, plant and equipment, net	162,172	152,461
Operating lease right-of-use assets	3,144	2,226
Other assets, net	450	415
Total assets	316,196	279,098
Current liabilities:
Accounts payable	5,433	4,592
Current portion of operating lease liabilities	899	1,187
Accrued expenses	4,856	5,809
Total current liabilities	11,188	11,588
Deferred tax liabilities, net	23,055	21,531
Operating lease liabilities, excluding current portion	2,311	1,030
Other liabilities	1,436	1,757
Total liabilities	37,990	35,906
Stockholders' equity:
Preferred stock, $5.00 par value; authorized 500,000 shares; none issued or outstanding
Common stock, $0.10 par value; 30,000,000 shares authorized and 6,681,469 issued at December 31, 2021; 15,000,000 authorized and 6,657,880 issued at December 31, 2020	669	666
Additional paid-in capital	31,774	29,457
Accumulated other comprehensive income
Retained earnings	301,611	268,186
Less treasury stock, 1,015,457 and 1,009,796 shares at December 31, 2021 and 2020, respectively, at cost	(55,848)	(55,117)
Total stockholders' equity	278,206	243,192
Total liabilities and stockholders' equity	$ 316,196	$ 279,098