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Financial instruments - Schedule of Assets and Liabilities measured at Fair Value (Details) - Recurring fair value measurement - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Long-term investments (Note 7) | Level 1 |  |
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items] |  |
Assets measured at fair value | $ 4,155 | $ 9,744
Long-term investments (Note 7) | Level 2 |  |
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items] |  |
Assets measured at fair value | 0 | 0
Fuel derivative contracts (Note 14) | Level 1 |  |
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items] |  |
Liabilities measured at fair value | 0 |
Fuel derivative contracts (Note 14) | Level 2 |  |
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items] |  |
Liabilities measured at fair value | (1,837) |
Interest rate swaps (Note 14) | Level 1 |  |
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items] |  |
Assets measured at fair value | 0 | 0
Interest rate swaps (Note 14) | Level 2 |  |
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items] |  |
Assets measured at fair value | 740 | 1,246
Fuel derivative contracts (Note 14) | Level 1 |  |
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items] |  |
Assets measured at fair value |  | 0
Fuel derivative contracts (Note 14) | Level 2 |  |
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items] |  |
Assets measured at fair value |  | 7,366
Convertible senior subordinated notes (Note 10) | Level 1 |  |
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items] |  |
Liabilities measured at fair value | 0 | 0
Convertible senior subordinated notes (Note 10) | Level 2 |  |
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items] |  |
Liabilities measured at fair value | 0 | (277,650)
Gold forward contracts (Note 14) | Level 1 |  |
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items] |  |
Liabilities measured at fair value | 0 | 0
Gold forward contracts (Note 14) | Level 2 |  |
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items] |  |
Liabilities measured at fair value | $ 0 | $ (4,952)

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- Definition The fair value of financial assets. [Refer: At fair value [member]; Financial assets] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2017-01-01 -Paragraph 25 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-01&anchor=para_25&doctype=Standard -URIDate 2017-03-09 + Details
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