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Financial instruments by category - Fair values based on recognized valuation methods (Details)
$ in Thousands
Dec. 31, 2017
USD ($)
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]  
Derivative financial assets (liabilities) $ 3,931
Options | Level 1  
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]  
Derivative financial assets (liabilities) 54
Futures | Level 1  
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]  
Derivative financial assets (liabilities) 3,911
Foreign-currency interest-rate swaps | Level 2  
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]  
Derivative financial assets (liabilities) $ (34)