Income and Mining Taxes - Income and Mining Tax (Expense) Benefit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Examination [Line Items]
Income tax (expense) benefit	$ 28,998	$ (33,247)	$ (29,075)
United States
Income Tax Examination [Line Items]
Deferred federal income tax expense (benefit)		6,367	(1,610)	$ (564)
United States - Alternative Minimum tax [Member]
Income Tax Examination [Line Items]
Current federal tax expense (benefit)		1,428	0	49
United States — State mining taxes
Income Tax Examination [Line Items]
Current federal tax expense (benefit)		(6,016)	(7,826)	(4,305)
Deferred federal income tax expense (benefit)		1,052	748	1,952
United States — Foreign withholding tax
Income Tax Examination [Line Items]
Current federal tax expense (benefit)		(8,466)	(1,838)	0
Argentina
Income Tax Examination [Line Items]
Current foreign tax expense (benefit)		55	10	715
Deferred foreign income tax expense (benefit)		1,531	115	(1,197)
Australia
Income Tax Examination [Line Items]
Current foreign tax expense (benefit)		0	14	130
Deferred foreign income tax expense (benefit)		0	(1,638)	3,223
Canada
Income Tax Examination [Line Items]
Current foreign tax expense (benefit)		876	(1,841)	(516)
Deferred foreign income tax expense (benefit)		104	1,338	2,875
Mexico
Income Tax Examination [Line Items]
Current foreign tax expense (benefit)		(30,763)	(9,581)	(476)
Deferred foreign income tax expense (benefit)		4,805	55,383	27,189
New Zealand
Income Tax Examination [Line Items]
Deferred foreign income tax expense (benefit)		$ 29	$ (27)	$ 0