Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015		Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Revenues		$ 176,247	$ 162,552		$ 506,641	$ 481,770
COSTS AND EXPENSES
Costs applicable to sales	[1]	105,408	120,237	[2]	307,428	354,397	[3]
Amortization		27,763	35,497		93,232	107,560
General and administrative		7,113	6,694		22,789	23,979
Exploration		3,706	2,112		7,669	9,957
Write-downs		0	0		4,446	0
Pre-development, reclamation, and other		4,491	4,938		13,059	13,968
Total costs and expenses		148,481	169,478		448,623	509,861
OTHER INCOME (EXPENSE), NET
Fair value adjustments, net		(961)	5,786		(13,235)	3,657
Interest expense, net of capitalized interest		(8,068)	(12,446)		(30,063)	(33,945)
Other, net		(3,635)	(8,893)		(4,178)	(14,257)
Total other income (expense), net		(12,664)	(15,553)		(47,476)	(44,545)
Income (loss) before income and mining taxes		15,102	(22,479)		10,542	(72,636)
Income and mining tax benefit (expense)		54,455	8,260		53,118	8,451
NET INCOME (LOSS)		69,557	(14,219)		63,660	(64,185)
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 ended September 30, 2016, respectively		1,387	(931)		4,533	(3,744)
Reclassification adjustments for impairment of equity securities		0	483		20	2,028
Reclassification adjustments for realized (gain) loss on sale of equity securities		(2,965)	0		(2,691)	904
Other comprehensive income (loss)		(1,578)	(448)		1,862	(812)
COMPREHENSIVE INCOME (LOSS)		$ 67,979	$ (14,667)		$ 65,522	$ (64,997)
NET INCOME (LOSS) PER SHARE
Basic (in dollars per share)		$ 0.43	$ (0.11)		$ 0.41	$ (0.52)
Diluted (in dollars per share)		$ 0.42	$ (0.11)		$ 0.40	$ (0.52)

[1] Excludes amortization.

[2] Excludes amortization

[3] Excludes amortization