Income and Mining Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Beginning unrecognized tax benefits	$ 3,776	$ 5,324
Gross increase to current period tax positions	0	0
Gross increase to prior period tax positions	137	37
Reductions in unrecognized tax benefits resulting from a lapse of the applicable statute of limitations	(1,207)	(1,585)
Ending unrecognized tax benefits	$ 2,706	$ 3,776