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Derivative financial instruments - Composition of open transactions (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
USD ($)
lb
Dec. 31, 2019
USD ($)
Disclosure of derivative financial instruments [Line Items]    
Quantity of commodity | lb 2,000  
Financial instruments designated as hedging instruments, at fair value $ 2,759 $ 0
Settlement period one [Member]    
Disclosure of derivative financial instruments [Line Items]    
Quantity of commodity | lb 1,000  
Future quotation $ 5,961  
Financial instruments designated as hedging instruments, at fair value 1,381  
Settlement period one [Member] | Bottom of range [member]    
Disclosure of derivative financial instruments [Line Items]    
Fixed Quotation $ 7,345  
Settlement period two [Member]    
Disclosure of derivative financial instruments [Line Items]    
Quantity of commodity | lb 1,000  
Future quotation $ 5,968  
Financial instruments designated as hedging instruments, at fair value 1,378  
Settlement period two [Member] | Bottom of range [member]    
Disclosure of derivative financial instruments [Line Items]    
Fixed Quotation $ 7,352