Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net income	$ 20,846	$ 11,451	$ 25,740
Adjustments to reconcile net income to net cash provided from operating activities:
Depreciation, depletion, and amortization	35,524	42,751	45,651
Amortization of deferred financing costs in interest expense	1,009	919	666
Stock-based compensation expense (non-cash)	5,313	4,957	3,174
Amortization of pension and post-retirement costs	5,551	4,865	4,060
Loss (gain) on sale of property, plant, and equipment	518	234	(648)
Deferred income tax (benefit) expense	(1,318)	20,256	(9,010)
Pension settlement charges	41,406	0	120
Changes in assets and liabilities net of acquired assets and liabilities:
Decrease (increase) in accounts receivable	(7,219)	(18,484)	(4,096)
Decrease (increase) in inventory	4,234	(9,462)	10,791
Decrease (increase) in prepaid and other current assets	1,162	(11,606)	658
Increase (decrease) in accounts payable and accrued expenses	8,820	34,433	2,758
Increase (decrease) in unearned revenue	477	4,336	(2,590)
Increase (decrease) in interest and taxes payable	435	(514)	2,511
Domestic pension plan contributions	(42,000)	(16,000)	(16,000)
Other — net	1,616	(341)	4,395
Net cash provided from operating activities	76,374	67,795	68,180
Cash flows from investing activities:
Payments for purchase of property, plant, and equipment	(27,702)	(27,516)	(27,177)
Payments for mine development	(6,558)	(1,560)	(9,861)
Payments for acquisition	0	(16,504)	(1,750)
Proceeds from sale of property, plant, and equipment	432	2,222	1,433
Net cash (used in) investing activities	(33,828)	(43,358)	(37,355)
Cash flows from financing activities:
Repayment of short-term debt	0	0	(8,305)
Proceeds from issuance of long-term debt	0	55,000	10,000
Repayment of long-term debt	(777)	(55,797)	(10,694)
Principal payments under capital lease obligations	(861)	(843)	(736)
Cash dividends paid	(8,389)	(7,913)	(7,496)
Deferred financing costs	0	(300)	(1,000)
Repurchase of common stock	(422)	(1,086)	(3,798)
Payments of withholding taxes for stock-based compensation awards	(3,156)	(4,506)	(1,089)
Net cash (used in) financing activities	(13,605)	(15,445)	(23,118)
Effects of exchange rate changes	(140)	1,388	(479)
Net change in cash and cash equivalents	28,801	10,380	7,228
Cash and cash equivalents at beginning of period	41,844	31,464	24,236
Cash and cash equivalents at end of period	$ 70,645	$ 41,844	$ 31,464