Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 11,451	$ 25,740	$ 32,158
Adjustments to reconcile net income to net cash provided from operating activities:
Depreciation, depletion, and amortization	42,751	45,651	37,817
Amortization of deferred financing costs in interest expense	919	666	654
Stock-based compensation expense (non-cash)	4,957	3,174	5,491
(Gain) loss on sale of property, plant, and equipment	(1,150)	(648)	768
Deferred tax (benefit) expense	20,256	(9,010)	4,368
Changes in assets and liabilities net of acquired assets and liabilities:
Decrease (increase) in accounts receivable	(18,484)	(4,096)	14,777
Decrease (increase) in inventory	(9,462)	10,791	19,372
Decrease (increase) in prepaid and other current assets	(11,606)	658	2,139
Increase (decrease) in accounts payable and accrued expenses	34,433	2,758	(17,989)
Increase (decrease) in unearned revenue	4,336	(2,590)	(1,184)
Increase (decrease) in interest and taxes payable	(514)	2,511	(910)
Increase (decrease) in long-term liabilities	(4,264)	(684)	(8,923)
Other — net	(5,828)	(6,741)	2,472
Net cash provided from operating activities	67,795	68,180	91,010
Cash flows from investing activities:
Payments for purchase of property, plant, and equipment	(27,516)	(27,177)	(29,505)
Payments for mine development	(1,560)	(9,861)	(22,585)
Payments for acquisition	(16,504)	(1,750)	0
Proceeds from sale of property, plant, and equipment	2,222	1,433	58
Net cash (used in) investing activities	(43,358)	(37,355)	(52,032)
Cash flows from financing activities:
Repayment of short-term debt	0	(8,305)	(653)
Proceeds from issuance of long-term debt	55,000	10,000	78,000
Repayment of long-term debt	(55,797)	(10,694)	(88,000)
Principal payments under capital lease obligations	(843)	(736)	(759)
Cash dividends paid	(7,913)	(7,496)	(7,132)
Deferred financing costs	(300)	(1,000)	(838)
Repurchase of common stock	(1,086)	(3,798)	(7,129)
Payments of withholding taxes for stock-based compensation awards	(4,506)	(1,089)	(366)
Net cash (used in) financing activities	(15,445)	(23,118)	(26,877)
Effects of exchange rate changes	1,388	(479)	(1,015)
Net change in cash and cash equivalents	10,380	7,228	11,086
Cash and cash equivalents at beginning of period	31,464	24,236	13,150
Cash and cash equivalents at end of period	$ 41,844	$ 31,464	$ 24,236