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Financial Instruments - Exposure to Currency Risk (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Cash and short-term investments $ 175,953 $ 180,881 $ 133,963
Income taxes receivable (payable), current and non- current (9,140) (8,452)  
Accounts payable and accrued liabilities and non- current liabilities (82,478) (95,164)  
Deferred tax assets and liabilities (168,549) (169,136)  
Currency risk [member]      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Cash and short-term investments 41,446 24,597  
Other current and non-current assets 44,579 55,821  
Canadian Dollar      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Income taxes receivable (payable), current and non- current (713) (45)  
Accounts payable and accrued liabilities and non- current liabilities (348) (142)  
Deferred tax assets and liabilities 0 (356)  
Canadian Dollar | Currency risk [member]      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Cash and short-term investments 25,062 6,513  
Other current and non-current assets 529 338  
Mexican Peso      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Income taxes receivable (payable), current and non- current (242) 5,884  
Accounts payable and accrued liabilities and non- current liabilities (26,013) (45,388)  
Deferred tax assets and liabilities (141,870) (150,394)  
Mexican Peso | Currency risk [member]      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Cash and short-term investments 5,188 9,416  
Other current and non-current assets 22,809 29,079  
Argentinian Peso      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Income taxes receivable (payable), current and non- current 837 367  
Accounts payable and accrued liabilities and non- current liabilities (28,685) (27,245)  
Deferred tax assets and liabilities 0 0  
Argentinian Peso | Currency risk [member]      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Cash and short-term investments 4,239 3,485  
Other current and non-current assets 19,720 24,062  
Bolivian Boliviano      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Income taxes receivable (payable), current and non- current (3,840) (3,365)  
Accounts payable and accrued liabilities and non- current liabilities (13,954) (13,476)  
Deferred tax assets and liabilities (10,076) (8,464)  
Bolivian Boliviano | Currency risk [member]      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Cash and short-term investments 4,659 4,329  
Other current and non-current assets 495 184  
European Euro      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Income taxes receivable (payable), current and non- current (780) (262)  
Accounts payable and accrued liabilities and non- current liabilities 0 0  
Deferred tax assets and liabilities 0 (53)  
European Euro | Currency risk [member]      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Cash and short-term investments 24 37  
Other current and non-current assets 0 0  
Peruvian Nuevo Sol      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Income taxes receivable (payable), current and non- current (4,402) (11,031)  
Accounts payable and accrued liabilities and non- current liabilities (13,478) (8,913)  
Deferred tax assets and liabilities (16,603) (9,867)  
Peruvian Nuevo Sol | Currency risk [member]      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Cash and short-term investments 2,274 817  
Other current and non-current assets $ 1,026 $ 2,158