Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
OPERATING ACTIVITIES:
Net income	$ 27,148	$ 17,754	$ 12,886
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	16,549	16,141	16,033
Amortization of deferred financing costs	15	14	29
Deferred income taxes	(7,612)	648	(227)
Loss on disposition of property, plant and equipment	377	622	39
Stock-based compensation	1,405	1,036	1,199
Changes in operating assets and liabilities:
Trade receivables, net	308	(892)	1,555
Inventories, net	(1,113)	2,295	(1,292)
Prepaid expenses and other current assets	(1,594)	308	698
Other assets	5	4	(47)
Accounts payable and accrued expenses	(1,130)	532	198
Other liabilities	(76)	(615)	1,423
Net cash provided by operating activities	34,282	37,847	32,494
INVESTING ACTIVITIES:
Purchase of property, plant and equipment	(21,337)	(17,664)	(11,459)
Acquisition of assets of a business		(50)	(50)
Proceeds from sale of property, plant and equipment	592	208	449
Net cash used in investing activities	(20,745)	(17,506)	(11,060)
FINANCING ACTIVITIES:
Repayments of term loans			(16,667)
Cash dividends paid	(3,013)	(2,782)	(2,798)
Proceeds from exercise of stock options	73	155	28
Purchase of treasury shares	(309)	(2,928)	(403)
Net cash used in financing activities	(3,249)	(5,555)	(19,840)
Net increase in cash and cash equivalents	10,288	14,786	1,594
Cash and cash equivalents at beginning of period	74,712	59,926	58,332
Cash and cash equivalents at end of period	$ 85,000	$ 74,712	$ 59,926