Unaudited Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Revenues	$ 31,200	$ 28,300
Cost of sales	25,776	23,742
Gross profit	5,424	4,558
Earnings of unconsolidated operations	15,555	14,955
Operating expenses
Selling, general and administrative expenses	10,627	9,520
Amortization of intangible assets	684	587
Gain on sale of assets	(53)	(400)
Operating expenses	11,258	9,707
Operating profit	9,721	9,806
Other expense (income)
Interest expense	646	932
Income from other unconsolidated affiliates	(315)	(308)
Closed mine obligations	379	383
Other, net, including interest income	31	(20)
Other (income) expense	741	987
Income from continuing operations before income tax provision	8,980	8,819
Income tax provision from continuing operations	804	599
Income from continuing operations	8,176	8,220
Discontinued operations, net of tax expense of $1,071 in the three months ended March 31, 2017	0	(3,242)
Net income	$ 8,176	$ 4,978
Basic earnings (loss) per share:
Continuing operations (in dollars per share)	$ 1.19	$ 1.21
Discontinued operations (in dollars per share)	0.00	(0.48)
Basic earnings per share (in dollars per share)	1.19	0.73
Diluted earnings (loss) per share:
Continuing operations (in dollars per share)	1.18	1.20
Discontinued operations (in dollars per share)	0.00	(0.47)
Diluted earnings per share (in dollars per share)	1.18	0.73
Dividends per share (in dollars per share)	$ 0.1650	$ 0.2675
Basic weighted average shares outstanding (in shares)	6,894	6,806
Diluted weighted average shares outstanding (in shares)	6,939	6,843