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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) |

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