Income and Mining Taxes - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
Tax (expense) benefit	$ (11,129)	$ (16,780)	$ 28,998
Unrecognized Tax Benefits	2,700	3,800	4,300
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	2,300	$ 3,500	$ 4,800
Minimum
Income Tax Contingency [Line Items]
Unrecognized income tax liability	2,500
Maximum
Income Tax Contingency [Line Items]
Unrecognized income tax liability	$ 0