Income and Mining Taxes - Valuation Allowance (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Income Tax Examination [Line Items]
Valuation allowance	$ 371,277	$ 272,839
United States
Income Tax Examination [Line Items]
Valuation allowance	213,783	223,444
Canada
Income Tax Examination [Line Items]
Valuation allowance	118,738	7,661
Mexico
Income Tax Examination [Line Items]
Valuation allowance	15,884	17,606
New Zealand
Income Tax Examination [Line Items]
Valuation allowance	21,863	22,482
Other jurisdictions
Income Tax Examination [Line Items]
Valuation allowance	$ 1,009	$ 1,646