Discontinued Operations - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
	Dec. 22, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash provided by (used in) operating activities, discontinued operations		$ 0	$ 5,175	$ (2,690)	$ 16,510
Cash provided by (used in) investing activities, discontinued operations		0	(375)	(28,470)	(763)
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	$ 7,100
Value added tax receivable, outstanding	12,700
Promissory note receivable	26,900
Expected post-closing obligations	$ 5,700
Cash provided by (used in) operating activities, discontinued operations			5,175	(2,690)	16,510
Cash provided by (used in) investing activities, discontinued operations			(375)	(28,470)	(763)
Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax		$ 0	$ 0	$ 1,525	$ 0