Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Revenues	$ 214,585	$ 159,919	$ 149,540	$ 185,554	$ 139,204	$ 148,762	$ 156,822	$ 127,109	$ 709,598		$ 571,897		$ 561,407
COSTS AND EXPENSES
Costs applicable to sales	121,982	101,559	102,229	114,490	84,903	84,594	81,820	84,058	440,260	[1]	335,375	[1]	403,827	[1]
Amortization	44,722	32,401	30,733	38,693	28,625	26,040	35,653	26,210	146,549		116,528		125,953
General and administrative									33,616		29,275		32,636
Exploration	7,454	9,792	7,813	5,252	5,260	3,706	2,233	1,731	30,311		12,930		11,521
Write-downs									0		4,446		246,625
Pre-development, reclamation, and other									18,936		14,411		16,204
Total costs and expenses									669,672		512,965		836,766
OTHER INCOME (EXPENSE), NET
Gain (Loss) on Extinguishment of Debt									(9,342)		(21,365)		15,916
Fair value adjustments, net, pretax									(864)		(11,581)		5,202
Interest expense, net of capitalized interest									(16,440)		(36,896)		(44,978)
Other, net									26,643		98		(17,667)
Total other income (expense), net									(3)		(69,744)		(41,527)
Total									39,923		(10,812)		(316,886)
Income and mining tax benefit (expense)									(28,998)		33,247		29,075
Income (loss) from continuing operations	14,349	(11,728)	(9,995)	18,299	(10,328)	46,123	8,280	(21,640)	10,925		22,435		(287,811)
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	(6,724)	(4,924)	(960)	364	2,020	23,436	6,217	1,244	(12,244)		32,917		(79,372)
Net income (loss)	$ 7,625	$ (16,652)	$ (10,955)	$ 18,663	$ (8,308)	$ 69,559	$ 14,497	$ (20,396)	(1,319)		55,352		(367,183)
OTHER COMPREHENSIVE INCOME (LOSS), Net of Tax:
Unrealized gain (loss) on equity securities, net of tax of ($767) for the year ended December 31, 2016									3,227		3,222		(4,154)
Reclassification adjustments for impairment of equity securities									426		703		2,346
Reclassification adjustments for realized (gain) loss on sale of equity securities									1,354		(2,691)		894
Other comprehensive income (loss)									5,007		1,234		(914)
COMPREHENSIVE INCOME (LOSS)									$ 3,688		$ 56,586		$ (368,097)
Income (Loss) from Continuing Operations, Per Basic Share	$ 0.08	$ (0.06)	$ (0.05)	$ 0.10	$ (0.06)	$ 0.29	$ 0.05	$ (0.15)	$ 0.06		$ 0.14		$ (2.22)
Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Basic Share	(0.04)	(0.03)	(0.01)	0.00	0.01	0.14	0.04	0.01	(0.07)		0.21		(0.61)
NET INCOME (LOSS) PER SHARE
Basic (in dollars per share)	0.04	(0.09)	(0.06)	0.10	(0.05)	0.43	0.09	(0.14)	(0.01)		0.35		(2.83)
Income (Loss) from Continuing Operations, Per Diluted Share	0.04	(0.09)	(0.06)	0.10	(0.05)	0.42	0.09	(0.14)	0.06		0.14		(2.22)
Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Diluted Share	0.08	(0.06)	(0.05)	0.10	(0.06)	0.28	0.05	(0.15)	(0.07)		0.20		(0.61)
Diluted (in dollars per share)	$ (0.04)	$ (0.03)	$ (0.01)	$ 0.00	$ 0.01	$ 0.14	$ 0.04	$ 0.01	$ (0.01)		$ 0.34		$ (2.83)

[1] Excludes amortization.