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Financial instruments and risk management - Summary of impact of hedging instrument on statement of financial position (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
Aluminium embedded derivatives separated from the power contract [Member]
Disclosure Of Financial Instruments [Line Items]
Notional amount	$ 1,786
Carrying value	8
Change in fair value used for measuring ineffectiveness for the period	205
Aluminium embedded derivatives separated from the power contract [Member] | Othe financial assets [Member]
Disclosure Of Financial Instruments [Line Items]
Line item in the statement of financial position	54
Aluminium embedded derivatives separated from the power contract [Member] | Other financial liability [Member]
Disclosure Of Financial Instruments [Line Items]
Line item in the statement of financial position	46
Highly Probable Forecast Aluminum Sales
Disclosure Of Financial Instruments [Line Items]
Change in fair value used for measuring ineffectiveness for the period	(182)
Cash flow hedge reserve US$m	$ 179

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- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

+ References

No definition available.

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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
-Note Effective 2018-01-01
-Name IFRS
-Number 7
-IssueDate 2017-01-01
-Paragraph 24B
-Subparagraph a
-Clause iv
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-01&anchor=para_24B_a_iv&doctype=Standard
-URIDate 2017-03-09

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Note Effective 2018-01-01
-Name IFRS
-Number 7
-IssueDate 2017-01-01
-Paragraph 24B
-Subparagraph b
-Clause i
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-01&anchor=para_24B_b_i&doctype=Standard
-URIDate 2017-03-09

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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 2018-01-01
-Name IFRS
-Number 7
-IssueDate 2017-01-01
-Paragraph 24
-Subparagraph a
-Clause i
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-02&anchor=para_24_a_i&doctype=Standard&book=B
-URIDate 2017-03-09

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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 2017-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_112_c&doctype=Standard
-URIDate 2017-03-09

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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
-Note Effective 2018-01-01
-Name IFRS
-Number 7
-IssueDate 2017-01-01
-Paragraph 24B
-Subparagraph b
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-01&anchor=para_24B_b_ii&doctype=Standard
-URIDate 2017-03-09

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Carrying value of hedging instrument.

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- Definition

Hedging instrument included in statement of financial position.

+ References

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