Income and Mining Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Beginning unrecognized tax benefits	$ 7,157	$ 2,131
Gross increase to current period tax positions	202	239
Gross increase to prior period tax positions	316	5,187
Reductions in unrecognized tax benefits resulting from a lapse of the applicable statute of limitations	(2,351)	(400)
Ending unrecognized tax benefits	$ 5,324	$ 7,157