Income and Mining Taxes - Valuation Allowance (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Tax Examination [Line Items]
Valuation allowance	$ 282,868	$ 338,539
United States
Income Tax Examination [Line Items]
Valuation allowance	235,395	292,446
Argentina
Income Tax Examination [Line Items]
Valuation allowance	3,914	6,197
Canada
Income Tax Examination [Line Items]
Valuation allowance	2,455	1,296
Mexico
Income Tax Examination [Line Items]
Valuation allowance	17,087	13,033
New Zealand
Income Tax Examination [Line Items]
Valuation allowance	23,792	23,717
Other
Income Tax Examination [Line Items]
Valuation allowance	$ 225	$ 1,850