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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)

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- Definition The fair value of financial instruments designated as hedging instruments. Hedging instruments are designated derivatives or (for a hedge of the risk of changes in foreign currency exchange rates only) designated non-derivative financial assets or non-derivative financial liabilities whose fair value or cash flows are expected to offset changes in the fair value or cash flows of a designated hedged item. [Refer: At fair value [member]; Derivatives [member]; Derivative financial assets; Derivative financial liabilities; Financial instruments, class [member]; Financial assets; Financial liabilities] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Note Expiry date 2021-01-01 -Name IFRS -Number 7 -IssueDate 2018-01-01 -Paragraph 22 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-02&anchor=para_22_b&doctype=Standard&book=b -URIDate 2018-03-16 + Details
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