Supplemental Guarantor Information Condensed Consolidated Statements of Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Revenues		$ 163,267	$ 185,554
Costs applicable to sales	[1]	99,340	114,490
Amortization		30,777	38,693
General and Administrative Expense		8,804	10,125
Exploration		6,683	5,252
Pre-development, reclamation, and other		4,225	3,837
Total costs and expenses		149,829	172,397
Gain (loss) on derivative instruments		4,987	(1,200)
Other, net		180	20,799
Interest expense, net		(5,965)	(3,579)
Total other income (expense), net		(798)	16,020
Total		12,640	29,177
Income and mining tax benefit (expense)		(11,949)	(10,878)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		691	18,299
Income (Loss) from Equity Method Investments		0	0
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent		550	364
Net income (loss)		1,241	18,663
Unrealized gain (loss) on available for sale securities		(278)	(2,182)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax		0	121
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax		0	1,471
Other comprehensive income (loss)		(278)	(590)
COMPREHENSIVE INCOME (LOSS)		963	18,073
Revenue		163,267	185,554
Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Revenues		93,230	107,194
Costs applicable to sales		68,245	71,202
Amortization		14,205	18,104
General and Administrative Expense		3	24
Exploration		2,245	1,727
Pre-development, reclamation, and other		1,947	1,781
Total costs and expenses		86,645	92,838
Gain (loss) on derivative instruments		(292)	(1,200)
Other, net		(137)	5,458
Interest expense, net		(353)	(175)
Total other income (expense), net		(782)	4,083
Total		5,803	18,439
Income and mining tax benefit (expense)		(1,120)	(2,434)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		4,683	16,005
Income (Loss) from Equity Method Investments		(38)	70
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent		(284)	0
Net income (loss)		4,361	16,075
Unrealized gain (loss) on available for sale securities		0	(279)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax			121
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax			(369)
Other comprehensive income (loss)			(527)
COMPREHENSIVE INCOME (LOSS)		4,361	15,548
Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Revenues		70,037	78,360
Costs applicable to sales		31,095	43,288
Amortization		16,326	20,265
General and Administrative Expense		4	(5)
Exploration		3,979	3,189
Pre-development, reclamation, and other		1,872	1,881
Total costs and expenses		53,276	68,618
Gain (loss) on derivative instruments		0	0
Other, net		(106)	1,533
Interest expense, net		(4,248)	(1,539)
Total other income (expense), net		(4,354)	(6)
Total		12,407	9,736
Income and mining tax benefit (expense)		(12,467)	(10,032)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		(60)	(296)
Income (Loss) from Equity Method Investments		(170)	(67)
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent		(175)	364
Net income (loss)		(405)	1
Unrealized gain (loss) on available for sale securities		0	0
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax			0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax			0
Other comprehensive income (loss)			0
COMPREHENSIVE INCOME (LOSS)		(405)	1
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Revenues		0	0
Costs applicable to sales		0	0
Amortization		246	324
General and Administrative Expense		8,797	10,106
Exploration		459	336
Pre-development, reclamation, and other		406	175
Total costs and expenses		9,908	10,941
Gain (loss) on derivative instruments		5,279	0
Other, net		4,142	15,222
Interest expense, net		(5,083)	(3,279)
Total other income (expense), net		4,338	11,943
Total		(5,570)	1,002
Income and mining tax benefit (expense)		1,638	1,588
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		(3,932)	2,590
Income (Loss) from Equity Method Investments		4,164	16,073
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent		1,009	0
Net income (loss)		1,241	18,663
Unrealized gain (loss) on available for sale securities		(278)	(2,182)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax			121
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax			1,471
Other comprehensive income (loss)			(590)
COMPREHENSIVE INCOME (LOSS)		963	18,073
Eliminations
Condensed Financial Statements, Captions [Line Items]
Revenues		0	0
Costs applicable to sales		0	0
Amortization		0	0
General and Administrative Expense		0	0
Exploration		0	0
Pre-development, reclamation, and other		0	0
Total costs and expenses		0	0
Gain (loss) on derivative instruments		0	0
Other, net		(3,719)	(1,414)
Interest expense, net		3,719	1,414
Total other income (expense), net		0	0
Total		0	0
Income and mining tax benefit (expense)		0	0
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		0	0
Income (Loss) from Equity Method Investments		(3,956)	(16,076)
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent		0	0
Net income (loss)		(3,956)	(16,076)
Unrealized gain (loss) on available for sale securities		0	279
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax			(121)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax			369
Other comprehensive income (loss)			527
COMPREHENSIVE INCOME (LOSS)		$ (3,956)	$ (15,549)

[1] Excludes amortization.