Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Revenues		$ 163,267	$ 185,554
COSTS AND EXPENSES
Costs applicable to sales	[1]	99,340	114,490
Amortization		30,777	38,693
General and administrative		8,804	10,125
Exploration		6,683	5,252
Pre-development, reclamation, and other		4,225	3,837
Total costs and expenses		149,829	172,397
OTHER INCOME (EXPENSE), NET
Fair value adjustments, net, pretax		4,987	(1,200)
Interest expense, net of capitalized interest		(5,965)	(3,579)
Other, net		180	20,799
Total other income (expense), net		(798)	16,020
Total		12,640	29,177
Income and mining tax benefit (expense)		(11,949)	(10,878)
Income (loss) from continuing operations		691	18,299
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent		550	364
Net income (loss)		1,241	18,663
OTHER COMPREHENSIVE INCOME (LOSS), Net of Tax:
Unrealized gain (loss) on debt and equity securities		(278)	(2,182)
Reclassification adjustments for impairment of equity securities		0	121
Reclassification adjustments for realized (gain) loss on sale of equity securities		0	1,471
Other comprehensive income (loss)		(278)	(590)
COMPREHENSIVE INCOME (LOSS)		$ 963	$ 18,073
Basic EPS
Income (Loss) from Continuing Operations, Per Basic Share		$ 0.004	$ 0.10
Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Basic Share		0.003	0.00
Earnings Per Share, Basic		0.007	0.10
Diluted EPS
Income (Loss) from Continuing Operations, Per Diluted Share		0.004	0.10
Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Diluted Share		0.003	0.00
Earnings Per Share, Diluted		$ 0.007	$ 0.10

[1] Excludes amortization.