Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 54,260	$ 67,218
Trade receivables, net	22,948	19,602
Inventories, net	13,388	12,846
Prepaid expenses and other current assets	2,139	1,692
Total current assets	92,735	101,358
Property, plant and equipment:
Mineral reserves and land	36,423	33,637
Proved natural gas properties, successful-efforts method	17,484	18,414
Buildings and building and leasehold improvements	7,251	5,814
Machinery and equipment	304,379	286,173
Furniture and fixtures	981	981
Automotive equipment	3,837	3,453
Property, plant and equipment	370,355	348,472
Less accumulated depreciation and depletion	(219,668)	(205,708)
Property, plant and equipment, net	150,687	142,764
Operating lease right-of-use assets	3,192
Other assets, net	423	549
Total assets	247,037	244,671
Current liabilities:
Accounts payable	4,430	4,570
Current portion of operating lease liabilities	1,294
Accrued expenses	3,735	3,393
Total current liabilities	9,459	7,963
Deferred tax liabilities, net	17,218	12,365
Operating lease liabilities, excluding current portion	1,866
Other liabilities	1,362	1,376
Total liabilities	29,905	21,704
Stockholders' equity:
Preferred stock, $5.00 par value; authorized 500,000 shares; none issued or outstanding
Common stock	663	661
Additional paid-in capital	27,464	25,867
Accumulated other comprehensive loss	(1)	(13)
Retained earnings	243,566	250,568
Less treasury stock, at cost	(54,560)	(54,116)
Total stockholders' equity	217,132	222,967
Total liabilities and stockholders' equity	$ 247,037	$ 244,671