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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