Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Revenue		$ 148,795	$ 159,920	$ 482,049	$ 495,014
COSTS AND EXPENSES
Costs applicable to sales	[1]	116,857	101,559	324,443	318,278
Amortization		31,184	32,400	91,420	101,827
General and administrative		7,729	7,345	24,183	24,495
Exploration		8,157	9,791	21,269	22,856
Pre-development, reclamation, and other		8,121	5,030	15,966	12,952
Total costs and expenses		172,048	156,125	477,281	480,408
OTHER INCOME (EXPENSE), NET
Loss on debt extinguishments		0	0	0	(9,342)
Fair value adjustments, net, pretax		715	0	2,907	(864)
Interest expense, net of capitalized interest		(5,818)	(3,595)	(17,801)	(10,918)
Other, net		(20,903)	2,361	(19,846)	27,134
Total other income (expense), net		(26,006)	(1,234)	(34,740)	6,010
Total		(49,259)	2,561	(29,972)	20,616
Income and mining tax (expense) benefit		(3,785)	(14,289)	(19,451)	(24,040)
Income (loss) from continuing operations		(53,044)	(11,728)	(49,423)	(3,424)
Income (loss) from discontinued operations		0	(4,924)	550	(5,520)
NET INCOME (LOSS)		(53,044)	(16,652)	(48,873)	(8,944)
OTHER COMPREHENSIVE INCOME (LOSS), Net of Tax:
Unrealized gain (loss) on debt and equity securities		192	1,066	(173)	(1,134)
Reclassification adjustments for impairment of equity securities		0	0	0	426
Reclassification adjustments for realized (gain) loss on sale of equity securities		0	32	0	1,300
Other comprehensive income (loss)		192	1,098	(173)	592
COMPREHENSIVE INCOME (LOSS)		$ (52,852)	$ (15,554)	$ (49,046)	$ (8,352)
Basic EPS
Income (Loss) from Continuing Operations, Per Basic Share		$ (0.286)	$ (0.065)	$ (0.267)	$ (0.019)
Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Basic Share		0.000	(0.028)	0.003	(0.031)
Earnings Per Share, Basic	[2]	(0.286)	(0.093)	(0.264)	(0.050)
Diluted EPS
Income (Loss) from Continuing Operations, Per Diluted Share		(0.286)	(0.065)	(0.267)	(0.019)
Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Diluted Share		0.000	(0.028)	0.003	(0.031)
Earnings Per Share, Diluted	[2]	$ (0.286)	$ (0.093)	$ (0.264)	$ (0.050)

[1] Excludes amortization.

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