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Financial instruments by category - Financial assets and financial liabilities measured at fair value (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]    
Derivative financial instruments $ 6,286 $ 4,483
Derivative financial liabilities (283) (552)
Level 1    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]    
Derivative financial instruments 6,286 4,463
Derivative financial liabilities (254) (498)
Level 2    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]    
Derivative financial instruments 0 20
Derivative financial liabilities $ (29) $ (54)