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Derivative financial instruments (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
USD ($)
lb
Dec. 31, 2017
USD ($)
lb
Disclosure of derivative financial instruments [Line Items]    
Quantity of commodity | lb 2,000 36,000
Financial instruments designated as hedging instruments, at fair value $ 2,759 $ 28,705
Settlement period one [Member]    
Disclosure of derivative financial instruments [Line Items]    
Quantity of commodity | lb 1,000 3,000
Fixed Quotation $ 7,345  
Future quotation 5,961 $ 7,275
Financial instruments designated as hedging instruments, at fair value $ 1,381 (3,787)
Settlement period one [Member] | Top of range [member]    
Disclosure of derivative financial instruments [Line Items]    
Fixed Quotation   6,050
Settlement period one [Member] | Bottom of range [member]    
Disclosure of derivative financial instruments [Line Items]    
Fixed Quotation   $ 5,972
Settlement period two [Member]    
Disclosure of derivative financial instruments [Line Items]    
Quantity of commodity | lb 1,000 3,000
Fixed Quotation $ 7,352  
Future quotation 5,968 $ 7,260
Financial instruments designated as hedging instruments, at fair value 1,378 (3,736)
Settlement period two [Member] | Top of range [member]    
Disclosure of derivative financial instruments [Line Items]    
Fixed Quotation   6,050
Settlement period two [Member] | Bottom of range [member]    
Disclosure of derivative financial instruments [Line Items]    
Fixed Quotation   $ 5,972
Settlement period three [Member]    
Disclosure of derivative financial instruments [Line Items]    
Quantity of commodity | lb   3,000
Future quotation   $ 7,247
Financial instruments designated as hedging instruments, at fair value   (3,693)
Settlement period three [Member] | Top of range [member]    
Disclosure of derivative financial instruments [Line Items]    
Fixed Quotation   6,050
Settlement period three [Member] | Bottom of range [member]    
Disclosure of derivative financial instruments [Line Items]    
Fixed Quotation   $ 5,972
Settlement period four [Member]    
Disclosure of derivative financial instruments [Line Items]    
Quantity of commodity | lb   3,000
Future quotation   $ 7,259
Financial instruments designated as hedging instruments, at fair value   (3,973)
Settlement period four [Member] | Top of range [member]    
Disclosure of derivative financial instruments [Line Items]    
Fixed Quotation   6,050
Settlement period four [Member] | Bottom of range [member]    
Disclosure of derivative financial instruments [Line Items]    
Fixed Quotation   $ 5,805
Settlement period five [Member]    
Disclosure of derivative financial instruments [Line Items]    
Quantity of commodity | lb   3,000
Future quotation   $ 7,269
Financial instruments designated as hedging instruments, at fair value   (3,484)
Settlement period five [Member] | Top of range [member]    
Disclosure of derivative financial instruments [Line Items]    
Fixed Quotation   6,300
Settlement period five [Member] | Bottom of range [member]    
Disclosure of derivative financial instruments [Line Items]    
Fixed Quotation   $ 5,900
Settlement period six [Member]    
Disclosure of derivative financial instruments [Line Items]    
Quantity of commodity | lb   3,000
Future quotation   $ 7,277
Financial instruments designated as hedging instruments, at fair value   (3,468)
Settlement period six [Member] | Top of range [member]    
Disclosure of derivative financial instruments [Line Items]    
Fixed Quotation   6,325
Settlement period six [Member] | Bottom of range [member]    
Disclosure of derivative financial instruments [Line Items]    
Fixed Quotation   $ 5,900
Settlement period seven [Member]    
Disclosure of derivative financial instruments [Line Items]    
Quantity of commodity | lb   3,000
Future quotation   $ 7,285
Financial instruments designated as hedging instruments, at fair value   (3,359)
Settlement period seven [Member] | Top of range [member]    
Disclosure of derivative financial instruments [Line Items]    
Fixed Quotation   6,350
Settlement period seven [Member] | Bottom of range [member]    
Disclosure of derivative financial instruments [Line Items]    
Fixed Quotation   $ 5,960
Settlement period eight [Member]    
Disclosure of derivative financial instruments [Line Items]    
Quantity of commodity | lb   3,000
Fixed Quotation 6,520  
Future quotation   $ 7,290
Financial instruments designated as hedging instruments, at fair value   $ (2,288)
Settlement period nine [Member]    
Disclosure of derivative financial instruments [Line Items]    
Quantity of commodity | lb   3,000
Fixed Quotation 7,100  
Future quotation   $ 7,296
Financial instruments designated as hedging instruments, at fair value   $ (580)
Settlement period ten [Member]    
Disclosure of derivative financial instruments [Line Items]    
Quantity of commodity | lb   3,000
Fixed Quotation 7,200  
Future quotation   $ 7,300
Financial instruments designated as hedging instruments, at fair value   $ (296)
Settlement period eleven [Member]    
Disclosure of derivative financial instruments [Line Items]    
Quantity of commodity | lb   3,000
Fixed Quotation 7,300  
Future quotation   $ 7,305
Financial instruments designated as hedging instruments, at fair value   $ (13)
Settlement period twele [Member]    
Disclosure of derivative financial instruments [Line Items]    
Quantity of commodity | lb   3,000
Fixed Quotation $ 7,300  
Future quotation   $ 7,309
Financial instruments designated as hedging instruments, at fair value   $ (28)