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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 |  |  |

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