Supplemental Guarantor Information Condensed Consolidated Statements of Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Revenues	$ 214,585	$ 159,919	$ 149,540	$ 185,554	$ 139,204	$ 148,762	$ 156,822	$ 127,109	$ 709,598		$ 571,897		$ 561,407
Costs applicable to sales	121,982	101,559	102,229	114,490	84,903	84,594	81,820	84,058	440,260	[1]	335,375	[1]	403,827	[1]
Amortization	44,722	32,401	30,733	38,693	28,625	26,040	35,653	26,210	146,549		116,528		125,953
General and Administrative Expense									33,616		29,275		32,636
Exploration	7,454	9,792	7,813	5,252	5,260	3,706	2,233	1,731	30,311		12,930		11,521
Write-downs									0		4,446		246,625
Pre-development, reclamation, and other									18,936		14,411		16,204
Total costs and expenses									669,672		512,965		836,766
Gain (Loss) on Extinguishment of Debt									(9,342)		(21,365)		15,916
Gain (loss) on derivative instruments									(864)		(11,581)		5,202
Other, net									26,643		98		(17,667)
Interest expense, net									(16,440)		(36,896)		(44,978)
Total other income (expense), net									(3)		(69,744)		(41,527)
Total									39,923		(10,812)		(316,886)
Income and mining tax benefit (expense)									(28,998)		33,247		29,075
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest									10,925		22,435		(287,811)
Income (Loss) from Equity Method Investments									0		0		0
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	(6,724)	(4,924)	(960)	364	2,020	23,436	6,217	1,244	(12,244)		32,917		(79,372)
Net income (loss)	$ 7,625	$ (16,652)	$ (10,955)	$ 18,663	$ (8,308)	$ 69,559	$ 14,497	$ (20,396)	(1,319)		55,352		(367,183)
Unrealized gain (loss) on available for sale securities									3,227		3,222		(4,154)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax									426		703		2,346
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax									1,354		(2,691)		894
Other comprehensive income (loss)									5,007		1,234		(914)
COMPREHENSIVE INCOME (LOSS)									3,688		56,586		(368,097)
Revenue									709,598		568,617		554,557
Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Revenues									433,050		423,488		378,278
Costs applicable to sales									293,340		252,836		261,830
Amortization									71,340		77,392		83,325
General and Administrative Expense									28		250		35
Exploration									13,689		6,127		3,931
Write-downs											0		1,630
Pre-development, reclamation, and other									7,497		5,839		5,920
Total costs and expenses									385,894		342,444		356,671
Gain (Loss) on Extinguishment of Debt									0		0		0
Gain (loss) on derivative instruments									(864)		(4,133)		818
Other, net									2,936		2,139		(3,106)
Interest expense, net									(975)		(861)		(966)
Total other income (expense), net									1,097		(2,855)		(3,254)
Total									48,253		78,189		18,353
Income and mining tax benefit (expense)									(5,758)		(7,517)		(2,354)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest									42,495		70,672		15,999
Income (Loss) from Equity Method Investments									(577)		(4,353)		(14,814)
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent									0		0		0
Net income (loss)									41,918		66,319		1,185
Unrealized gain (loss) on available for sale securities									915		3,156		(3,118)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax									426		703		2,346
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax									486		(3,181)		894
Other comprehensive income (loss)									1,827		678		122
COMPREHENSIVE INCOME (LOSS)									43,745		66,997		1,307
Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Revenues									276,548		148,409		183,129
Costs applicable to sales									146,920		82,539		141,997
Amortization									74,052		37,578		40,637
General and Administrative Expense									209		321		196
Exploration									15,030		5,207		5,325
Write-downs											4,446		244,995
Pre-development, reclamation, and other									6,734		6,528		6,201
Total costs and expenses									242,945		136,619		439,351
Gain (Loss) on Extinguishment of Debt									0		0		0
Gain (loss) on derivative instruments									0		(5,813)		3,160
Other, net									10,179		(1,314)		(15,121)
Interest expense, net									(8,534)		(5,961)		(7,921)
Total other income (expense), net									1,645		(13,088)		(19,882)
Total									35,248		(1,298)		(276,104)
Income and mining tax benefit (expense)									(25,410)		29,031		29,602
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest									9,838		27,733		(246,502)
Income (Loss) from Equity Method Investments									4,416		0		0
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent									(12,244)		32,917		(79,372)
Net income (loss)									2,010		60,650		(325,874)
Unrealized gain (loss) on available for sale securities									0		0		0
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax									0		0		0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax									0		0		0
Other comprehensive income (loss)									0		0		0
COMPREHENSIVE INCOME (LOSS)									2,010		60,650		(325,874)
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Revenues									0		0		0
Costs applicable to sales									0		0		0
Amortization									1,157		1,558		1,991
General and Administrative Expense									33,379		28,704		32,405
Exploration									1,592		1,596		2,265
Write-downs											0		0
Pre-development, reclamation, and other									4,705		2,044		4,083
Total costs and expenses									40,833		33,902		40,744
Gain (Loss) on Extinguishment of Debt									(9,342)		(21,365)		15,916
Gain (loss) on derivative instruments									0		(1,635)		1,224
Other, net									21,254		4,357		4,336
Interest expense, net									(14,657)		(35,158)		(39,867)
Total other income (expense), net									(2,745)		(53,801)		(18,391)
Total									(43,578)		(87,703)		(59,135)
Income and mining tax benefit (expense)									2,170		11,733		1,827
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest									(41,408)		(75,970)		(57,308)
Income (Loss) from Equity Method Investments									40,089		131,322		(309,875)
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent									0		0		0
Net income (loss)									(1,319)		55,352		(367,183)
Unrealized gain (loss) on available for sale securities									3,227		3,222		(4,154)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax									426		703		2,346
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax									1,354		(2,691)		894
Other comprehensive income (loss)									5,007		1,234		(914)
COMPREHENSIVE INCOME (LOSS)									3,688		56,586		(368,097)
Eliminations
Condensed Financial Statements, Captions [Line Items]
Revenues									0		0		0
Costs applicable to sales									0		0		0
Amortization									0		0		0
General and Administrative Expense									0		0		0
Exploration									0		0		0
Write-downs											0		0
Pre-development, reclamation, and other									0		0		0
Total costs and expenses									0		0		0
Gain (Loss) on Extinguishment of Debt									0		0		0
Gain (loss) on derivative instruments									0		0		0
Other, net									(7,726)		(5,084)		(3,776)
Interest expense, net									7,726		5,084		3,776
Total other income (expense), net									0		0		0
Total									0		0		0
Income and mining tax benefit (expense)									0		0		0
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest									0		0		0
Income (Loss) from Equity Method Investments									(43,928)		(126,969)		324,689
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent									0		0		0
Net income (loss)									(43,928)		(126,969)		324,689
Unrealized gain (loss) on available for sale securities									(915)		(3,156)		3,118
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax									(426)		(703)		(2,346)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax									(486)		3,181		(894)
Other comprehensive income (loss)									(1,827)		(678)		(122)
COMPREHENSIVE INCOME (LOSS)									$ (45,755)		$ (127,647)		$ 324,567

[1] Excludes amortization.