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Financial instruments - Narrative (Details)
 12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2018
EUR (€)
Dec. 31, 2018
USD ($)
Jul. 07, 2017
USD ($)
Jun. 01, 2017
USD ($)
Dec. 31, 2016
USD ($)
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]              
Cash and cash equivalents   $ 147,468,000   $ 102,752,000     $ 144,671,000
Cash flows from (used in) operating activities $ 450,867,000 155,000,000          
Borrowings outstanding   702,181,000   479,535,000      
Revolving credit facility:              
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]              
Line of credit facility, maximum borrowing capacity       500,000,000 $ 500,000,000    
Borrowings outstanding   344,149,000   395,822,000      
Undrawn borrowing facilities       100,000,000      
Fekola Equipment Loan Facility              
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]              
Line of credit facility, maximum borrowing capacity | €     € 71,400,000.0        
Borrowings outstanding   50,569,000   57,242,000      
Undrawn borrowing facilities     € 2,000,000 1,900,000      
Masbate Equipment Loan Facility              
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]              
Line of credit facility, maximum borrowing capacity           $ 18,000,000  
Borrowings outstanding   7,719,000   13,071,000      
Undrawn borrowing facilities       1,000,000      
Currency risk              
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]              
Risk exposure associated with instruments sharing characteristic       $ 79,000,000      
Reasonably possible change in risk variable, percent 10.00%            
Sensitivity analysis for types of market risk, reasonably possible change in risk variable, impact on cash and cash equivalents $ 2,000,000            
Fuel derivative contracts | Commodity price risk              
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]              
Reasonably possible change in risk variable, percent 10.00%            
Sensitivity analysis for types of market risk, reasonably possible change in risk variable, impact on financial liabilities $ 3,000,000            
Nicaraguan Cordoba Versus United States Dollar              
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]              
Change in foreign currency value     (5.00%) (5.00%)      
Principal | Revolving credit facility:              
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]              
Borrowings outstanding   $ 350,000,000   $ 400,000,000      
Principal | Fekola Equipment Loan Facility              
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]              
Borrowings outstanding       58,634,000      
Principal | Masbate Equipment Loan Facility              
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]              
Borrowings outstanding       $ 13,526,000