Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Revenues		$ 206,138	$ 148,387
COSTS AND EXPENSES
Costs applicable to sales	[1]	132,712	101,555
Amortization		40,104	27,964
General and administrative		10,133	8,276
Exploration		5,252	1,731
Write-downs		0	4,446
Pre-development, reclamation, and other		4,581	4,204
Total costs and expenses		192,782	148,176
OTHER INCOME (EXPENSE), NET
Fair value adjustments, net		(1,200)	(8,695)
Interest expense, net of capitalized interest		(3,586)	(11,120)
Other, net		21,139	1,314
Total other income (expense), net		16,353	(18,501)
Income (loss) before income and mining taxes		29,709	(18,290)
Income and mining tax benefit (expense)		(11,046)	(2,106)
NET INCOME (LOSS)		18,663	(20,396)
OTHER COMPREHENSIVE INCOME (LOSS), Net of Tax:
Unrealized gain (loss) on equity securities, net of tax of ($1,101) for the three months ended March 31, 2016		(2,182)	1,043
Reclassification adjustments for impairment of equity securities		121	0
Reclassification adjustments for realized (gain) loss on sale of equity securities		1,471	588
Other comprehensive income (loss)		(590)	1,631
COMPREHENSIVE INCOME (LOSS)		$ 18,073	$ (18,765)
NET INCOME (LOSS) PER SHARE
Basic (in dollars per share)		$ 0.10	$ (0.14)
Diluted (in dollars per share)		$ 0.10	$ (0.14)

[1] Excludes amortization.