Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 59,926	$ 58,332
Trade receivables, net	15,889	17,444
Inventories, net	14,728	13,436
Prepaid expenses and other current assets	1,418	2,116
Total current assets	91,961	91,328
Property, plant and equipment:
Mineral reserves and land	20,196	20,030
Proved natural gas properties, successful-efforts method	18,398	18,384
Buildings and building and leasehold improvements	5,523	5,466
Machinery and equipment	222,812	214,194
Furniture and fixtures	961	887
Automotive equipment	3,796	3,501
Property, plant and equipment	271,686	262,462
Less accumulated depreciation and depletion	(167,308)	(153,949)
Property, plant and equipment, net	104,378	108,513
Other assets, net	160	145
Total assets	196,499	199,986
Current liabilities:
Current installments of debt		16,667
Accounts payable	6,022	5,166
Accrued expenses	2,720	3,132
Total current liabilities	8,742	24,965
Deferred tax liabilities, net	19,184	18,825
Other liabilities	1,946	1,505
Total liabilities	$ 29,872	$ 45,295
Commitments and contingencies
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; 6,541,049 and 6,499,403 shares issued at December 31, 2015 and 2014, respectively	$ 655	$ 652
Additional paid-in capital	21,642	20,418
Accumulated other comprehensive loss		(1,024)
Retained earnings	194,798	184,710
Less treasury stock, at cost, 935,368 and 928,469 shares at December 31, 2015 and 2014, respectively	(50,468)	(50,065)
Total stockholders' equity	166,627	154,691
Total liabilities and stockholders' equity	$ 196,499	$ 199,986