Income and Mining Taxes - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Income Tax Contingency [Line Items]
Tax (expense) benefit	$ 218	$ 3,785	$ (13,986)	$ 19,451
Effective Income Tax Rate Reconciliation, Percent	1.60%	7.70%	15.60%	64.90%
Unrecognized Tax Benefits	$ 2,600		$ 2,600		$ 3,800
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	2,200		2,200		$ 3,700
Minimum
Income Tax Contingency [Line Items]
Unrecognized income tax liability	2,500		2,500
Maximum
Income Tax Contingency [Line Items]
Unrecognized income tax liability	$ 3,500		$ 3,500