Summary of Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue		$ 143,855	$ 148,795	$ 169,987	$ 163,267	$ 214,585	$ 159,919	$ 149,540	$ 185,554	$ 625,904	$ 709,598	$ 571,897
Pre-development, reclamation, and other						15,106	12,374	11,110	13,962	20,043	18,936	14,411
Amortization		37,053	31,184	29,459	30,777	44,722	32,401	30,733	38,693	128,473	146,549	116,528
Gross Profit		(9,705)	754	32,282	33,150	47,881	25,959	16,578	32,371
Other Cost and Expense, Operating		11,239	15,850	11,270	13,029
Income (loss) from continuing operations		468	(53,044)	2,930	691	14,349	(11,728)	(9,995)	18,299	(48,955)	10,925	22,435
Income (loss) from discontinued operations		0	0	0	550	(6,724)	(4,924)	(960)	364	550	(12,244)	32,917
NET INCOME (LOSS)		468	(53,044)	2,930	1,241	7,625	(16,652)	(10,955)	18,663	(48,405)	(1,319)	55,352
Cash provided by (used in) activities of continuing operations		72	5,789	(1,294)	15,541	91,811	37,308	24,103	43,938	20,108	197,160	96,461
Capital expenditures		$ (17,805)	$ (39,472)	$ (41,165)	$ (42,345)	$ (47,054)	$ (28,982)	$ (37,107)	$ (23,591)	$ (140,787)	$ (136,734)	$ (94,382)
Income (Loss) from Continuing Operations, Per Basic Share		$ 0.00	$ (0.290)	$ 0.020	$ 0.000	$ 0.08	$ (0.060)	$ (0.050)	$ 0.100	$ (0.260)	$ 0.060	$ 0.140
Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Basic Share		0.00	0.000	0.000	0.000	(0.04)	(0.030)	(0.010)	0.000	0.000	(0.070)	0.210
Earnings Per Share, Basic	[1]	0.000	(0.290)	0.020	0.000	0.040	(0.090)	(0.060)	0.100	(0.260)	(0.010)	0.350
Income (Loss) from Continuing Operations, Per Diluted Share		0.000	(0.290)	0.020	0.000	0.080	(0.060)	(0.050)	0.100	(0.260)	0.060	0.140
Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Diluted Share		0.000	0.000	0.000	0.000	(0.040)	(0.030)	(0.010)	0.000	0.000	(0.070)	0.200
Earnings Per Share, Diluted	[1]	$ 0.000	$ (0.290)	$ 0.020	$ 0.000	$ 0.040	$ (0.090)	$ (0.060)	$ 0.100	$ (0.260)	$ (0.010)	$ 0.340
Product [Member]
Costs applicable to sales	[2]	$ 116,507	$ 116,857	$ 108,246	$ 99,340	$ 121,982	$ 101,559	$ 102,229	$ 114,490	$ 440,950	$ 440,260	$ 335,375
Mineral, Exploration [Member]
Costs applicable to sales		$ 4,128	$ 8,157	$ 6,429	$ 6,683	$ 7,454	$ 9,792	$ 7,813	$ 5,252	$ 25,397	$ 30,311	$ 12,930

[1] Due to rounding, the sum of net income per share from continuing operations and discontinued operations may not equal net income per share.

[2] Excludes amortization.