Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenue - consolidated mines	$ 92,008	$ 104,953	$ 140,317
Revenue - royalty and other	12,770	6,128	7,681
Total revenues	104,778	111,081	147,998
Cost of sales - consolidated mines	85,657	96,374	156,092
Cost of sales - royalty and other	2,202	2,625	3,138
Total cost of sales	87,859	98,999	159,230
Gross profit (loss)	16,919	12,082	(11,232)
Earnings of unconsolidated mines	61,361	55,238	48,432
Operating expenses
Selling, general and administrative expenses	47,491	48,863	40,105
Centennial asset impairment charge	982	17,443	0
Amortization of intangible assets	2,123	2,503	2,606
(Gain) loss on sale of assets	(5,130)	170	(1,784)
Operating Expenses	45,466	68,979	40,927
Operating profit (loss)	32,814	(1,659)	(3,727)
Other expense (income)
Interest expense	3,440	4,318	4,962
Income from other unconsolidated affiliates	(1,246)	(1,221)	(2,040)
Closed mine obligations	1,590	(214)	919
Other, net, including interest income	(72)	2,151	(331)
Other (income) expense	3,712	5,034	3,510
Income (loss) from continuing operations before income tax provision (benefit)	29,102	(6,693)	(7,237)
Income tax provision (benefit) from continuing operations	639	(9,649)	(9,510)
Income from continuing operations	28,463	2,956	2,273
Discontinued operations, net of tax expense of $2,162, $14,512 and $12,325 in 2017, 2016 and 2015, respectively	1,874	26,651	19,711
Net income	$ 30,337	$ 29,607	$ 21,984
Basic earnings per share:
Continuing operations (USD per share)	$ 4.17	$ 0.43	$ 0.32
Discontinued operations (USD per share)	0.27	3.91	2.82
Basic earnings per share (USD per share)	4.44	4.34	3.14
Diluted earnings per share:
Continuing operations (USD per share)	4.14	0.43	0.32
Discontinued operations (USD per share)	0.27	3.89	2.81
Diluted earnings per share (USD per share)	$ 4.41	$ 4.32	$ 3.13
Basic weighted average shares outstanding (in shares)	6,830	6,818	7,001
Diluted weighted average shares outstanding (in shares)	6,873	6,854	7,022