Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 85,000	$ 74,712
Trade receivables, net	16,473	16,781
Inventories, net	13,546	12,433
Prepaid expenses and other current assets	2,996	1,110
Total current assets	118,015	105,036
Property, plant and equipment:
Mineral reserves and land	24,254	23,687
Proved natural gas properties, successful-efforts method	18,414	18,412
Buildings and building and leasehold improvements	5,643	5,611
Machinery and equipment	248,294	232,912
Furniture and fixtures	958	961
Automotive equipment	2,673	4,011
Property, plant and equipment	300,236	285,594
Less accumulated depreciation and depletion	(190,518)	(180,613)
Property, plant and equipment, net	109,718	104,981
Other assets, net	713	142
Total assets	228,446	210,159
Current liabilities:
Accounts payable	6,263	5,587
Accrued expenses	3,096	3,521
Total current liabilities	9,359	9,108
Deferred tax liabilities, net	12,374	19,832
Other liabilities	1,461	1,580
Total liabilities	23,194	30,520
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; 6,582,135 and 6,563,440 shares issued at December 31, 2017 and 2106, respectively	659	657
Additional paid-in capital	24,307	22,831
Accumulated other comprehensive income (loss)	86	(223)
Retained earnings	233,905	209,770
Less treasury stock, 993,314 and 989,300 shares at December 31, 2017 and 2016, respectively, at cost	(53,705)	(53,396)
Total stockholders' equity	205,252	179,639
Total liabilities and stockholders' equity	$ 228,446	$ 210,159