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Financial instruments by category - Financial assets and financial liabilities measured at fair value (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]    
Derivative financial instruments $ 4,483 $ 3,398
Derivative financial liabilities (552) (7,068)
Level 1    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]    
Derivative financial instruments 4,463 2,789
Derivative financial liabilities (498) (1,196)
Level 2    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]    
Derivative financial instruments 20 609
Derivative financial liabilities $ (54) $ (5,872)