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Financial instruments and risk management - Summary of impact of hedging instrument on statement of financial position (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018
Disclosure of detailed information about financial instruments [line items] |  |
Notional amount | $ 1,656 | $ 1,786
Carrying value | 58 | 88
Aluminium embedded derivatives separated from the power contract [Member] |  |
Disclosure of detailed information about financial instruments [line items] |  |
Notional amount | 1,656 | 1,786
Carrying value | 42 | 8
Change in fair value used for measuring ineffectiveness for the period | 29 | 205
Embedded derivatives in Other Financial Assets | 66 | 54
Embedded derivatives in Other Financial Liabilities | 24 | 46
Highly Probable Forecast Aluminum Sales |  |
Disclosure of detailed information about financial instruments [line items] |  |
Cash flow hedge reserve US$m | 196 | 179
Change in fair value used for measuring ineffectiveness for the period | (50) | (182)
Highly Probable Forecast Aluminum Sales | Cash flow hedge reserve |  |
Disclosure of detailed information about financial instruments [line items] |  |
Total hedging gain recognised in OCI US$m | 36 | 181
Gain (loss) on hedge ineffectiveness recognised in profit or loss | (7) | 24
Amount reclassified to other comprehensive income from profit or loss applying overlay approach, net of tax | $ 19 | $ 2

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- Definition The amount recognised in other comprehensive income on reclassification from profit or loss when applying the overlay approach, net of tax. [Refer: Other comprehensive income] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Note Effective on first application of IFRS 9 -Name IFRS -Number 4 -IssueDate 2019-01-01 -Paragraph 35D -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=4&code=ifrs-tx-2019-en-b&anchor=para_35D_b&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition The amount of current derivative financial assets. [Refer: Derivative financial assets] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2019-01-01 -Paragraph 55 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_55&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition The amount of derivative financial assets held for hedging. [Refer: Derivative financial assets] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2019-01-01 -Paragraph 55 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_55&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition The amount of derivative financial liabilities held for hedging. [Refer: Derivative financial liabilities] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2019-01-01 -Paragraph 55 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_55&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition The gain (loss) on the change in fair value of the hedged item used as a basis for recognising hedge ineffectiveness. Hedge ineffectiveness is the extent to which the changes in the fair value or the cash flows of the hedging instrument are greater or less than those on the hedged item. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2019-01-01 -Paragraph 24B -Subparagraph a -Clause iv -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2019-en-r&anchor=para_24B_a_iv&doctype=Standard -URIDate 2019-03-27 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2019-01-01 -Paragraph 24B -Subparagraph b -Clause i -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2019-en-r&anchor=para_24B_b_i&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition The gain (loss) recognised in other comprehensive income on hedge ineffectiveness. [Refer: Gain (loss) on hedge ineffectiveness] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2019-01-01 -Paragraph 24C -Subparagraph a -Clause i -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2019-en-r&anchor=para_24C_a_i&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition The gain (loss) recognised in profit or loss on hedge ineffectiveness. [Refer: Gain (loss) on hedge ineffectiveness] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2019-01-01 -Paragraph 24C -Subparagraph a -Clause i -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2019-en-r&anchor=para_24C_a_i&doctype=Standard -URIDate 2019-03-27 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2019-01-01 -Paragraph 24C -Subparagraph b -Clause ii -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2019-en-r&anchor=para_24C_b_ii&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition The gains (losses) on hedging instruments in fair value hedges. [Refer: Fair value hedges [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Note Expiry date 2021-01-01 -Name IFRS -Number 7 -IssueDate 2019-01-01 -Paragraph 24 -Subparagraph a -Clause i -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2017-en-b&anchor=para_24_a_i&doctype=Standard&book=b -URIDate 2019-03-27 + Details
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- Definition The nominal or face amount of a financial instrument, used to calculate payments made on that instrument. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2019-01-01 -Paragraph 112 -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_112_c&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition A component of equity representing the reserve of cash flow hedges in relation to continuing hedges. [Refer: Reserve of cash flow hedges] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2019-01-01 -Paragraph 24B -Subparagraph b -Clause ii -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2019-en-r&anchor=para_24B_b_ii&doctype=Standard -URIDate 2019-03-27 + Details
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- Details
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