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Financial instruments disclosures - Summary of financial instruments carried at fair value (Parenthetical) (Detail) - USD ($) | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2018 | Dec. 31, 2017
Disclosure Of Financial Instruments [Line Items] |  |
Provisionally priced receivables recognised relating to sales contracts | $ 658,000,000 |
Transfers between Level 1 and Level 2 | 0 | $ 0
Transfers between Level 1 and Level 2 | $ 0 | 0
Aluminium forward contracts/options embedded in electricity purchase contracts [member] | Level 3 [member] |  |
Disclosure Of Financial Instruments [Line Items] |  |
Long-term embedded derivatives expiry term | Level 3 financial instruments, under IFRS 9, primarily consist of electricity purchase contracts linked to the LME with terms expiring between 2020 and 2030 |
Long-term embedded derivatives with fair net value | $ 135,000,000 | $ (98,000,000)

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