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Derivative financial instruments - Additional information (Details) - USD ($)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of derivative financial instruments [Line Items]      
Hedge ineffectiveness $ 0 $ 0 $ 0
Fair value of the hedge contracts 0 2,759,000  
Gain/loss on hedge ineffectiveness recognised in profit or loss 4,300,000 (1,400,000) $ (10,900,000)
Net deferred income tax asset 240,386,000 209,401,000  
Trade account receivable due from related parties 36,100,000 19,300,000  
Trade accounts receivable net $ 600,000 800,000  
Other reserves [member]      
Disclosure of derivative financial instruments [Line Items]      
Net deferred income tax asset   $ 1,946,000