Income and Mining Taxes - Valuation Allowance (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Income Tax Examination [Line Items]
Valuation allowance	$ 401,304	$ 371,277
United States
Income Tax Examination [Line Items]
Valuation allowance	215,396	213,783
Canada
Income Tax Examination [Line Items]
Valuation allowance	146,611	118,738
Mexico
Income Tax Examination [Line Items]
Valuation allowance	15,885	15,884
New Zealand
Income Tax Examination [Line Items]
Valuation allowance	22,740	21,863
Other jurisdictions
Income Tax Examination [Line Items]
Valuation allowance	$ 672	$ 1,009