Income and Mining Taxes - Valuation Allowance (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Income Tax Examination [Line Items]
Valuation allowance	$ 272,839	$ 282,868
United States
Income Tax Examination [Line Items]
Valuation allowance	223,444	235,395
Canada
Income Tax Examination [Line Items]
Valuation allowance	7,661	2,455
Mexico
Income Tax Examination [Line Items]
Valuation allowance	17,606	17,087
New Zealand
Income Tax Examination [Line Items]
Valuation allowance	22,482	23,792
Other
Income Tax Examination [Line Items]
Valuation allowance	$ 1,646	$ 4,139