Income and Mining Taxes - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Income Tax Contingency [Line Items]
Tax (expense) benefit	$ (8,658)	$ 11,949
Effective Income Tax Rate Reconciliation, Percent	25.80%	94.50%
Unrecognized Tax Benefits	$ 3,100		$ 3,800
Minimum
Income Tax Contingency [Line Items]
Unrecognized income tax liability	3,500
Maximum
Income Tax Contingency [Line Items]
Unrecognized income tax liability	$ 4,500