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Financial instruments by category - Fair values based on recognized valuation methods (Details)
$ in Thousands
Dec. 31, 2018
USD ($)
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]  
Derivative financial assets (liabilities) $ 6,003
Level 1 | Futures  
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]  
Derivative financial assets (liabilities) 5,790
Level 1 | Options  
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]  
Derivative financial assets (liabilities) 242
Level 2 | NDF  
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]  
Derivative financial assets (liabilities) $ (29)