Unaudited Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Revenues	$ 21,941	$ 32,402	$ 78,341	$ 85,778
Cost of sales	19,466	30,755	66,711	75,926
Gross profit	2,475	1,647	11,630	9,852
Earnings of unconsolidated mines	16,197	15,102	44,627	40,785
Operating expenses
Selling, general and administrative expenses	11,723	10,765	31,809	30,786
Centennial asset impairment charge	0	17,443	0	17,443
(Gain) loss on sale of assets	(475)	502	(3,500)	1,424
Amortization of intangible assets	435	818	1,641	1,936
Operating expenses	11,683	29,528	29,950	51,589
Operating profit (loss)	6,989	(12,779)	26,307	(952)
Other expense (income)
Interest expense	946	1,036	2,806	3,182
Income from other unconsolidated affiliates	(313)	(307)	(932)	(913)
Closed mine obligations	336	223	1,071	948
Other, net, including interest income	64	(10)	15	2,229
Other (income) expense	1,033	942	2,960	5,446
Income (loss) from continuing operations before income tax provision (benefit)	5,956	(13,721)	23,347	(6,398)
Income tax provision (benefit) from continuing operations	2,625	(11,589)	4,564	(13,970)
Income (loss) from continuing operations	3,331	(2,132)	18,783	7,572
Discontinued operations, net of tax expense of $236 and $2,655 in the three and nine months ended September 30, 2017, respectively, and net of tax expense of $11,044 and $12,966 in the three and nine months ended September 30, 2016, respectively.	5,067	1,691	1,381	(2,096)
Net income (loss)	$ 8,398	$ (441)	$ 20,164	$ 5,476
Basic earnings (loss) per share:
Continuing operations (in dollars per share)	$ 0.49	$ (0.31)	$ 2.75	$ 1.11
Discontinued operations (in dollars per share)	0.74	0.25	0.20	(0.31)
Basic earnings per share (in dollars per share)	1.23	(0.06)	2.95	0.80
Diluted earnings (loss) per share:
Continuing operations (in dollars per share)	0.49	(0.31)	2.74	1.10
Discontinued operations (in dollars per share)	0.74	0.25	0.20	(0.30)
Diluted earnings per share (in dollars per share)	1.23	(0.06)	2.94	0.80
Dividends per share (in dollars per share)	$ 0.2725	$ 0.2675	$ 0.8125	$ 0.7975
Basic weighted average shares outstanding (in shares)	6,839	6,786	6,825	6,831
Diluted weighted average shares outstanding (in shares)	6,866	6,786	6,854	6,858