Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 74,712	$ 59,926
Trade receivables, net	16,781	15,889
Inventories, net	12,433	14,728
Prepaid expenses and other current assets	1,110	1,418
Total current assets	105,036	91,961
Property, plant and equipment:
Mineral reserves and land	23,687	20,196
Proved natural gas properties, successful-efforts method	18,412	18,398
Buildings and building and leasehold improvements	5,611	5,523
Machinery and equipment	232,912	222,812
Furniture and fixtures	961	961
Automotive equipment	4,011	3,796
Property, plant and equipment	285,594	271,686
Less accumulated depreciation and depletion	(180,613)	(167,308)
Property, plant and equipment, net	104,981	104,378
Other assets, net	142	160
Total assets	210,159	196,499
Current liabilities:
Accounts payable	5,587	6,022
Accrued expenses	3,521	2,720
Total current liabilities	9,108	8,742
Deferred tax liabilities, net	19,832	19,184
Other liabilities	1,580	1,946
Total liabilities	30,520	29,872
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; 6,563,440 and 6,541,049 shares issued at December 31, 2016 and 2015, respectively	657	655
Additional paid-in capital	22,831	21,642
Accumulated other comprehensive loss	(223)
Retained earnings	209,770	194,798
Less treasury stock, at cost 989,300 and 935,368 shares at December 31, 2016 and 2015, respectively	(53,396)	(50,468)
Total stockholders' equity	179,639	166,627
Total liabilities and stockholders' equity	$ 210,159	$ 196,499