Assets and Liabilities Held For Sale - Statements (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
CURRENT ASSETS
Current assets	$ 91,421				$ 71,442				$ 91,421	$ 71,442
CURRENT LIABILITIES
Current liabilities	50,677				15,470				50,677	15,470
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Write-downs									3,390
Income (loss) from discontinued operations.	(6,724)	$ (4,924)	$ (960)	$ 364	2,020	$ 23,436	$ 6,217	$ 1,244	(12,244)	32,917	$ (79,372)
Cash provided by (used in) operating activities, discontinued operations									11,296	29,356	26,130
Cash provided by (used in) investing activities, discontinued operations									(1,392)	(6,631)	(6,220)
Held-for-sale
CURRENT ASSETS
Cash and cash equivalents	32,931				43,870				32,931	43,870
Receivables	7,295				7,016				7,295	7,016
Inventory	10,655				12,590				10,655	12,590
Prepaid expenses and other	13,415				7,966				13,415	7,966
Property, plant and equipment, net	20,240				0				20,240	0
Mining properties, net	6,885				0				6,885	0
Current assets	91,421				71,442				91,421	71,442
NON-CURRENT ASSETS
Property, plant and equipment, net	0				23,373				0	23,373
Mining properties, net	0				8,165				0	8,165
Receivables	0				17,225				0	17,225
TOTAL ASSETS	91,421				120,205				91,421	120,205
CURRENT LIABILITIES
Accounts payable	10,974				8,675				10,974	8,675
Accrued liabilities and other	5,161				6,382				5,161	6,382
Reclamation	15,179				413				15,179	413
Other	19,363				0				19,363	0
Current liabilities	50,677				15,470				50,677	15,470
NON-CURRENT LIABILITIES
Reclamation	0				10,212				0	10,212
Deferred tax liabilities	0				4,987				0	4,987
Other	0				18,558				0	18,558
TOTAL LIABILITIES	$ 50,677				$ 49,227				50,677	49,227
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Revenue									73,065	93,880	84,679
Costs applicable to sales									74,074	74,166	75,827
Amortization									5,899	6,633	17,798
General and administrative									172	101	198
Exploration									23	0	126
Write-downs									3,390	0	66,712
Pre-development, reclamation, and other									4,664	2,808	1,589
Interest expense, net of capitalized interest									(27)	(24)	(725)
Other, net									1,763	1,777	1,736
Pretax profit or loss of discontinued operations related to major classes of pretax profit (loss)									(13,421)	11,925	(76,560)
Pretax gain or loss on the disposal of the discontinued operation									0	0	0
Total pretax gain or loss on discontinued operations									(13,421)	11,925	(76,560)
Income and mining tax (expense) benefit									1,177	20,992	(2,812)
Cash provided by (used in) operating activities, discontinued operations									11,296	29,356	26,130
Cash provided by (used in) investing activities, discontinued operations									$ (1,392)	$ (6,631)	$ (6,220)