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Financial instruments by category - Financial assets and financial liabilities measured at fair value (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]    
Derivative financial instruments $ 1,435 $ 6,286
Derivative financial liabilities (1,423) (283)
Level 1    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]    
Derivative financial instruments 1,257 6,286
Derivative financial liabilities (1,423) (254)
Level 2    
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]    
Derivative financial instruments 178 0
Derivative financial liabilities $ 0 $ (29)