Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2016		Jun. 30, 2015		Jun. 30, 2016		Jun. 30, 2015
Statement of Comprehensive Income [Abstract]
Revenue		$ 182,007		$ 166,263		$ 330,394		$ 319,219
COSTS AND EXPENSES
Costs applicable to sales	[1]	100,465	[2]	119,097	[3]	202,020	[4]	234,160	[5]
Amortization		37,505		38,974		65,470		72,064
General and administrative		7,400		8,451		15,676		17,286
Exploration		2,233		3,579		3,963		7,845
Write-downs		0		0		4,446		0
Pre-development, reclamation, and other		4,364		2,267		8,568		9,030
Total costs and expenses		151,967		172,368		300,143		340,385
OTHER INCOME (EXPENSE), NET
Fair value adjustments, net		(3,579)		2,754		(12,274)		(2,130)
Interest expense, net of capitalized interest		(10,875)		(10,734)		(21,995)		(21,499)
Other, net		(1,857)		(2,852)		(543)		(5,362)
Total other income (expense), net		(16,311)		(10,832)		(34,812)		(28,991)
Income (loss) before income and mining taxes		13,729		(16,937)		(4,561)		(50,157)
Income and mining tax benefit (expense)		768		260		(1,338)		192
NET INCOME (LOSS)		14,497		(16,677)		(5,899)		(49,965)
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 ended June 30, 2016, respectively, and $7 for the three months June 30, 2015		2,103		(1,312)		3,146		(2,813)
Reclassification adjustments for impairment of equity securities		20		31		20		1,545
Reclassification adjustments for realized (gain) loss on sale of equity securities		(314)		904		273		904
Other comprehensive income (loss)		1,809		(377)		3,439		(364)
COMPREHENSIVE INCOME (LOSS)		$ 16,306		$ (17,054)		$ (2,460)		$ (50,329)
NET INCOME (LOSS) PER SHARE
Basic (in dollars per share)		$ 0.09		$ (0.12)		$ (0.04)		$ (0.42)
Diluted (in dollars per share)		$ 0.09		$ (0.12)		$ (0.04)		$ (0.42)

[1] Excludes amortization.

[2] Segment assets include receivables, prepaids, inventories, property, plant and equipment, and mineral interest

[3] Excludes amortization

[4] Excludes amortization

[5] Excludes amortization