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Derivative financial instruments (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
USD ($)
lb
Dec. 31, 2016
USD ($)
lb
Disclosure of derivative financial instruments [Line Items]    
Quantity of commodity | lb 36,000 7,107
Fixed Quotation $ 7,300  
Financial instruments designated as hedging instruments, at fair value 28,705 $ (3,863)
Other reserves [member]    
Disclosure of derivative financial instruments [Line Items]    
Financial instruments designated as hedging instruments, at fair value $ 19,602,000 $ 2,723,000
Settlement period one [Member]    
Disclosure of derivative financial instruments [Line Items]    
Quantity of commodity | lb 3,000 2,542
Fixed Quotation   $ 4,917
Future quotation $ 7,275 5,526
Financial instruments designated as hedging instruments, at fair value (3,788) $ (1,548)
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 3,000 2,270
Fixed Quotation   $ 5,001
Future quotation $ 7,260 5,530
Financial instruments designated as hedging instruments, at fair value (3,736) $ (1,199)
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 1,795
Fixed Quotation   $ 4,860
Future quotation $ 7,247 5,535
Financial instruments designated as hedging instruments, at fair value (3,693) $ (1,208)
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 3,000 500
Fixed Quotation   $ 5,720
Future quotation $ 7,259 5,536
Financial instruments designated as hedging instruments, at fair value (3,973) $ 92
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  
Future quotation $ 7,309  
Financial instruments designated as hedging instruments, at fair value $ (27)