Income and Mining Taxes - Summary of Tax Credit Carryforwards (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Tax Credit Carryforward [Line Items]
Regular net operating losses	$ 650,936
Capital losses	87,431
Alternative minimum tax credits	793
Foreign tax credits	30,358
United States
Tax Credit Carryforward [Line Items]
Regular net operating losses	423,572
Capital losses	87,431
Alternative minimum tax credits	793
Foreign tax credits	30,358
Canada
Tax Credit Carryforward [Line Items]
Regular net operating losses	82,383
Capital losses	0
Alternative minimum tax credits	0
Foreign tax credits	0
Mexico
Tax Credit Carryforward [Line Items]
Regular net operating losses	58,688
Capital losses	0
Alternative minimum tax credits	0
Foreign tax credits	0
New Zealand
Tax Credit Carryforward [Line Items]
Regular net operating losses	82,378
Capital losses	0
Alternative minimum tax credits	0
Foreign tax credits	0
Other
Tax Credit Carryforward [Line Items]
Regular net operating losses	3,915
Capital losses	0
Alternative minimum tax credits	0
Foreign tax credits	$ 0