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Long-term debt - Revolving Credit Facilities (Details) |  |  |  | 11 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Jul. 07, 2017 USD ($) | Jun. 01, 2017 USD ($) payment | Feb. 13, 2017 | Nov. 30, 2017 USD ($) | Dec. 31, 2018 EUR (€) payment | Dec. 31, 2018 USD ($) payment | Dec. 31, 2017 EUR (€) | Dec. 31, 2017 USD ($) | Dec. 31, 2018 USD ($) | Jan. 01, 2018 USD ($) | May 30, 2017 USD ($)
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Principal |  |  |  |  |  |  |  | $ 702,181,000 | $ 479,535,000 |  |
Interest expense, capitalized to mineral property interests |  |  |  |  |  | $ 0 |  | 20,052,000 |  |  |
Repayments of lines of credit |  |  |  |  |  | 200,000,000 |  | 0 |  |  |
Repayment of credit facility |  |  |  |  |  | 27,670,000 |  | 16,782,000 |  |  |
Equipment loan facilities, drawdowns net of transaction costs (Note 10) |  |  |  |  |  | $ 32,117,000 |  | 68,224,000 |  |  |
Debt service reserve account |  |  |  |  |  |  |  | 12,918,000 | 13,736,000 |  |
Revolving credit facility: |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity | $ 500,000,000 |  |  |  |  |  |  |  | 500,000,000 |  |
Accordion feature higher borrowing capacity | $ 600,000,000 |  |  |  |  |  |  |  |  |  |
Borrowings maturity term | 4 years |  |  |  |  |  |  |  |  |  |
Principal |  |  |  |  |  |  |  | 344,149,000 | 395,822,000 |  |
Undrawn borrowing facilities |  |  |  |  |  |  |  |  | 100,000,000 |  |
Fekola Equipment Loan Facility |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity | € |  |  |  |  | € 71,400,000.0 |  |  |  |  |  |
Borrowings maturity term |  |  | 30 months |  | 5 years | 5 years |  |  |  |  |
Principal |  |  |  |  |  |  |  | 50,569,000 | 57,242,000 |  |
Undrawn borrowing facilities |  |  |  |  | € 2,000,000 |  |  |  | 1,900,000 |  |
Equipment loan facility, drawdowns net of transaction costs (Note 10) |  |  |  |  | € 20,000,000 | $ 24,000,000 | € 49,000,000 | 54,000,000 |  |  |
Mandatory reserve deposit payment coverage period |  |  |  |  | 6 months | 6 months |  |  |  |  |
Debt service reserve account |  |  |  |  | € 8,000,000 |  |  |  | $ 9,000,000 |  |
Number of quarterly installment payments | payment |  |  |  |  | 20 | 20 |  |  |  |  |
Commitment fee initial payment period |  |  |  |  | 24 months | 24 months |  |  |  |  |
Otjikoto Equipment Loan Facility |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Additional borrowing capacity |  |  |  |  |  |  |  |  |  |  | $ 6,000,000
Unused capacity commitment fee |  |  |  |  | 1.20% |  |  |  | 1.20% |  |
Principal |  |  |  |  |  |  |  | 20,750,000 | $ 10,816,000 |  |
Interest expense, capitalized to mineral property interests |  |  |  |  |  | $ 2,000,000 |  |  |  |  |
Equipment loan facility, drawdowns net of transaction costs (Note 10) |  |  |  |  |  | $ 6,000,000 |  |  |  |  |
Mandatory reserve deposit payment coverage period |  |  |  |  | 6 months | 6 months |  |  |  |  |
Debt service reserve account |  |  |  |  |  |  |  | 5,000,000 | 4,000,000 |  |
Number of quarterly installment payments | payment |  |  |  |  | 20 | 20 |  |  |  |  |
Masbate Equipment Loan Facility |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity |  | $ 18,000,000 |  |  |  |  |  |  |  |  |
Unused capacity commitment fee |  | 1.15% |  |  |  |  |  |  |  |  |
Borrowings maturity term |  | 5 years |  |  |  |  |  |  |  |  |
Principal |  |  |  |  |  |  |  | 7,719,000 | 13,071,000 |  |
Undrawn borrowing facilities |  |  |  |  |  |  |  |  | $ 1,000,000 |  |
Equipment loan facility, drawdowns net of transaction costs (Note 10) |  |  |  |  |  | $ 8,000,000 |  | 9,000,000 |  |  |
Number of quarterly installment payments | payment |  | 20 |  |  |  |  |  |  |  |  |
Bottom of range | Revolving credit facility: |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Unused capacity commitment fee | 0.50% |  |  |  |  |  |  |  |  |  |
Bottom of range | Fekola Equipment Loan Facility |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Principal | € |  |  |  |  | € 5,000,000 |  |  |  |  |  |
Bottom of range | Masbate Equipment Loan Facility |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Principal |  | $ 500,000 |  |  |  |  |  |  |  |  |
Top of range | Revolving credit facility: |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Unused capacity commitment fee | 0.925% |  |  |  |  |  |  |  |  |  |
Portion of convertible notes outstanding |  |  |  |  |  |  |  |  |  | $ 100,000,000 |
London Interbank Offered Rate LIBOR | Otjikoto Equipment Loan Facility |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Borrowings, adjustment to interest rate basis |  |  |  |  | 3.85% |  |  |  | 3.85% |  |
London Interbank Offered Rate LIBOR | Masbate Equipment Loan Facility |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Borrowings, adjustment to interest rate basis |  | 3.85% |  |  |  |  |  |  |  |  |
London Interbank Offered Rate LIBOR | Bottom of range | Revolving credit facility: |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Borrowings, adjustment to interest rate basis | 2.25% |  |  |  |  |  |  |  |  | 2.50% |
London Interbank Offered Rate LIBOR | Top of range | Revolving credit facility: |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Borrowings, adjustment to interest rate basis | 3.25% |  |  |  |  |  |  |  |  | 4.00% |
EURIBOR | Fekola Equipment Loan Facility |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Borrowings, adjustment to interest rate basis |  |  |  |  | 5.10% |  |  |  | 5.10% |  |
Fekola |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Interest expense, capitalized to mineral property interests |  |  |  | $ 20,000,000 |  |  |  |  |  |  |
Fekola | Revolving credit facility: |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Interest expense, capitalized to mineral property interests |  |  |  |  |  |  |  | 8,000,000 |  |  |
Tranche 1 | Fekola Equipment Loan Facility |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Unused capacity commitment fee |  |  |  |  | 1.15% |  |  |  | 1.15% |  |
Tranche 2 | Fekola Equipment Loan Facility |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Additional commitment fee percentage |  |  |  |  | 0.50% |  |  |  | 0.50% |  |
Tranche 3 | Fekola Equipment Loan Facility |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Additional commitment fee percentage |  |  |  |  | 0.4167% |  |  |  | 0.4167% |  |
Principal | Revolving credit facility: |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Principal |  |  |  |  |  |  |  | $ 350,000,000 | $ 400,000,000 |  |
Principal | Fekola Equipment Loan Facility |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Principal |  |  |  |  |  |  |  |  | 58,634,000 |  |
Principal | Otjikoto Equipment Loan Facility |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Principal |  |  |  |  |  |  |  |  | 10,922,000 |  |
Principal | Masbate Equipment Loan Facility |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |
Principal |  |  |  |  |  |  |  |  | $ 13,526,000 |  |

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