Income and Mining Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning unrecognized tax benefits	$ 2,706	$ 3,776
Gross increase to current period tax positions	0	0
Gross increase to prior period tax positions		137
Gross decrease to prior period tax positions	(122)
Reductions in unrecognized tax benefits resulting from a lapse of the applicable statute of limitations	(1,861)	(1,207)
Ending unrecognized tax benefits	$ 723	$ 2,706