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Derivative financial instruments - Gold forwards narrative (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Jun. 11, 2015
Disclosure of detailed information about financial instruments [line items] |  |  |
Unrealized gain (loss) on derivative instruments | $ (4,757) | $ 9,684 |
Novated Contracts Under Old RCF |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Unrealized gain (loss) on derivative instruments | $ 5,000 | $ 5,000 |
Not Designated As Hedging Instrument | Novated Contracts Under Old RCF |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Financial liabilities, at fair value |  |  | $ 14,000

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