Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 67,218	$ 85,000
Trade receivables, net	19,602	16,473
Inventories, net	12,846	13,546
Prepaid expenses and other current assets	1,692	2,996
Total current assets	101,358	118,015
Property, plant and equipment:
Mineral reserves and land	33,637	24,254
Proved natural gas properties, successful-efforts method	18,414	18,414
Buildings and building and leasehold improvements	5,814	5,643
Machinery and equipment	286,173	248,294
Furniture and fixtures	981	958
Automotive equipment	3,453	2,673
Property, plant and equipment	348,472	300,236
Less accumulated depreciation and depletion	(205,708)	(190,518)
Property, plant and equipment, net	142,764	109,718
Other assets, net	549	713
Total assets	244,671	228,446
Current liabilities:
Accounts payable	4,570	6,263
Accrued expenses	3,393	3,096
Total current liabilities	7,963	9,359
Deferred tax liabilities, net	12,365	12,374
Other liabilities	1,376	1,461
Total liabilities	21,704	23,194
Stockholders' equity:
Preferred stock, $5.00 par value; authorized 500,000 shares; none issued or outstanding
Common stock, $0.10 par value; authorized 15,000,000 shares; 6,606,100 and 6,582,135 shares issued at December 31, 2018 and 2017, respectively	661	659
Additional paid-in capital	25,867	24,307
Accumulated other comprehensive (loss) income	(13)	86
Retained earnings	250,568	233,905
Less treasury stock, 998,699 and 993,314 shares at December 31, 2018 and 2017, respectively, at cost	(54,116)	(53,705)
Total stockholders' equity	222,967	205,252
Total liabilities and stockholders' equity	$ 244,671	$ 228,446