Unaudited Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Revenues	$ 33,681	$ 28,100	$ 64,881	$ 56,400
Cost of sales	28,835	23,503	54,611	47,245
Gross profit	4,846	4,597	10,270	9,155
Earnings of unconsolidated operations	15,423	13,475	30,978	28,430
Operating expenses
Selling, general and administrative expenses	11,863	10,566	22,490	20,086
Amortization of intangible assets	814	619	1,498	1,206
Gain on sale of assets	(210)	(2,625)	(263)	(3,025)
Operating expenses	12,467	8,560	23,725	18,267
Operating profit	7,802	9,512	17,523	19,318
Other expense (income)
Interest expense	569	928	1,215	1,860
Income from other unconsolidated affiliates	(318)	(311)	(633)	(619)
Closed mine obligations	343	352	722	735
Other, net, including interest income	(373)	(29)	(342)	(49)
Other (income) expense	221	940	962	1,927
Income from continuing operations before income tax provision	7,581	8,572	16,561	17,391
Income tax provision from continuing operations	1,188	1,340	1,992	1,939
Income from continuing operations	6,393	7,232	14,569	15,452
Discontinued operations, net of tax expense of $2,443 and $3,514 in the three and six months ended June 30, 2017, respectively.	0	(444)	0	(3,686)
Net income	$ 6,393	$ 6,788	$ 14,569	$ 11,766
Basic earnings (loss) per share:
Continuing operations (in dollars per share)	$ 0.92	$ 1.06	$ 2.11	$ 2.27
Discontinued operations (in dollars per share)	0.00	(0.07)	0.00	(0.54)
Basic earnings per share (in dollars per share)	0.92	0.99	2.11	1.73
Diluted earnings (loss) per share:
Continuing operations (in dollars per share)	0.92	1.06	2.10	2.26
Discontinued operations (in dollars per share)	0.00	(0.07)	0.00	(0.54)
Diluted earnings per share (in dollars per share)	0.92	0.99	2.10	1.72
Dividends per share (in dollars per share)	$ 0.1650	$ 0.2725	$ 0.33	$ 0.5400
Basic weighted average shares outstanding (in shares)	6,940	6,835	6,914	6,818
Diluted weighted average shares outstanding (in shares)	6,940	6,850	6,939	6,847