Income and Mining Taxes - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Income Tax Contingency [Line Items]
Tax (expense) benefit	$ (5,546)	$ 3,717	$ (14,204)	$ 15,666
Unrecognized Tax Benefits	2,700		2,700		$ 3,800
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	$ 2,200		$ 2,200		$ 3,700