Income and Mining Taxes - Summary of Tax Credit Carryforwards (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Tax Credit Carryforward [Line Items]
Regular net operating losses	$ 565,365
Alternative minimum tax net operating losses	179,882
Capital losses	86,790
Alternative minimum tax credits	1,654
Foreign tax credits	41,730
United States
Tax Credit Carryforward [Line Items]
Regular net operating losses	369,973
Alternative minimum tax net operating losses	179,882
Capital losses	72,772
Alternative minimum tax credits	1,654
Foreign tax credits	41,730
Canada
Tax Credit Carryforward [Line Items]
Regular net operating losses	39,833
Alternative minimum tax net operating losses	0
Capital losses	14,018
Alternative minimum tax credits	0
Foreign tax credits	0
Mexico
Tax Credit Carryforward [Line Items]
Regular net operating losses	56,958
Alternative minimum tax net operating losses	0
Capital losses	0
Alternative minimum tax credits	0
Foreign tax credits	0
New Zealand
Tax Credit Carryforward [Line Items]
Regular net operating losses	86,165
Alternative minimum tax net operating losses	0
Capital losses	0
Alternative minimum tax credits	0
Foreign tax credits	0
Other
Tax Credit Carryforward [Line Items]
Regular net operating losses	12,436
Alternative minimum tax net operating losses	0
Capital losses	0
Alternative minimum tax credits	0
Foreign tax credits	$ 0