Assets and Liabilities Held For Sale - Narrative (Details) - USD ($) $ in Thousands		12 Months Ended
	Dec. 22, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Write-down of assets		$ 3,400
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
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net smelter returns royalty (percentage)	2.00%
Net smelter returns royalty, estimated value	$ 5,000
Value added tax receivable, outstanding	13,000
Promissory note receivable	28,000
Expected post-closing obligations	$ 6,000
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)