Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Revenues		$ 175,963	$ 176,247	$ 555,455	$ 506,641
COSTS AND EXPENSES
Costs applicable to sales	[1]	118,924	105,408	377,257	307,428
Amortization		33,830	27,763	106,880	93,232
General and administrative		7,412	7,113	24,587	22,789
Exploration		9,814	3,706	22,879	7,669
Write-downs		0	0	0	4,446
Pre-development, reclamation, and other		7,961	4,491	16,908	13,059
Total costs and expenses		177,941	148,481	548,511	448,623
OTHER INCOME (EXPENSE), NET
Gain (Loss) on Extinguishment of Debt		0	(10,040)	(9,342)	(10,040)
Fair value adjustments, net		0	(961)	(864)	(13,235)
Interest expense, net of capitalized interest		(3,606)	(8,068)	(10,941)	(30,063)
Other, net		3,164	6,405	28,439	5,862
Total other income (expense), net		(442)	(12,664)	7,292	(47,476)
Income (loss) before income and mining taxes		(2,420)	15,102	14,236	10,542
Income and mining tax benefit (expense)		(14,232)	54,455	(23,180)	53,118
NET INCOME (LOSS)		(16,652)	69,557	(8,944)	63,660
OTHER COMPREHENSIVE INCOME (LOSS), Net of Tax:
Unrealized gain (loss) on equity securities, net of tax of $997 and ($1,177) for the three and nine months September 30, 2016, respectively		1,066	1,387	(1,134)	4,533
Reclassification adjustments for impairment of equity securities		0	0	426	20
Reclassification adjustments for realized (gain) loss on sale of equity securities		32	(2,965)	1,300	(2,691)
Other comprehensive income (loss)		1,098	(1,578)	592	1,862
COMPREHENSIVE INCOME (LOSS)		$ (15,554)	$ 67,979	$ (8,352)	$ 65,522
NET INCOME (LOSS) PER SHARE
Basic (in dollars per share)		$ (0.09)	$ 0.43	$ (0.05)	$ 0.41
Diluted (in dollars per share)		$ (0.09)	$ 0.42	$ (0.05)	$ 0.40

[1] Excludes amortization