Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2017		Jun. 30, 2016		Jun. 30, 2017		Jun. 30, 2016
Statement of Comprehensive Income [Abstract]
Revenues	$ 173,354		$ 182,007		$ 379,492		$ 330,394
COSTS AND EXPENSES
Costs applicable to sales	125,621	[1]	100,465	[2]	258,333	[3]	202,020	[4]
Amortization	32,946		37,505		73,050		65,470
General and administrative	7,042		7,400		17,175		15,676
Exploration	7,813		2,233		13,065		3,963
Write-downs	0		0		0		4,446
Pre-development, reclamation, and other	4,366		4,364		8,947		8,568
Total costs and expenses	177,788		151,967		370,570		300,143
OTHER INCOME (EXPENSE), NET
Gain (Loss) on Extinguishment of Debt	(9,342)		0		(9,342)		0
Fair value adjustments, net	336		(3,579)		(864)		(12,274)
Interest expense, net of capitalized interest	(3,749)		(10,875)		(7,335)		(21,995)
Other, net	4,136		(1,857)		25,275		(543)
Total other income (expense), net	(8,619)		(16,311)		7,734		(34,812)
Income (loss) before income and mining taxes	(13,053)		13,729		16,656		(4,561)
Income and mining tax benefit (expense)	2,098		768		(8,948)		(1,338)
NET INCOME (LOSS)	(10,955)		14,497		7,708		(5,899)
OTHER COMPREHENSIVE INCOME (LOSS), Net of Tax:
Unrealized gain (loss) on equity securities, net of tax of ($1,164) and ($2,174) for the three and six months June 30, 2016, respectively	(18)		2,103		(2,200)		3,146
Reclassification adjustments for impairment of equity securities	305		20		426		20
Reclassification adjustments for realized (gain) loss on sale of equity securities	(203)		(314)		1,268		273
Other comprehensive income (loss)	84		1,809		(506)		3,439
COMPREHENSIVE INCOME (LOSS)	$ (10,871)		$ 16,306		$ 7,202		$ (2,460)
NET INCOME (LOSS) PER SHARE
Basic (in dollars per share)	$ (0.06)		$ 0.09		$ 0.04		$ (0.04)
Diluted (in dollars per share)	$ (0.06)		$ 0.09		$ 0.04		$ (0.04)

[1] Excludes amortization

[2] Excludes amortization

[3] Excludes amortization

[4] Excludes amortization