Financial instruments and risk management - Summary of effect of the cash flow hedge in statement of profit or loss and other comprehensive income (OCI) (Detail) - Cash flow hedge reserve [member] - Highly Probable Forecast Aluminum Sales $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($)
Disclosure Of Financial Instruments [Line Items]
Total hedging gain recognised in OCI US$m	$ 181
Ineffectiveness recognised in profit or loss US$m	24
Amount reclassified from OCI to profit or loss US$m	$ 2
Net operating costs [Member]
Disclosure Of Financial Instruments [Line Items]
Line item in the statement of profit or loss	Net operating costs (raw materials, consumables, repairs and maintenance)
Consolidated sales revenue [Member]
Disclosure Of Financial Instruments [Line Items]
Line item in the statement of profit or loss	Consolidated sales revenue

Financial instruments and risk management - Summary of effect of the cash flow hedge in statement of profit or loss and other comprehensive income (OCI) (Detail) - Cash flow hedge reserve [member] - Highly Probable Forecast Aluminum Sales
$ in Millions

12 Months Ended

Dec. 31, 2018

USD ($)

Disclosure Of Financial Instruments [Line Items]

Total hedging gain recognised in OCI US$m

$ 181

Ineffectiveness recognised in profit or loss US$m

Amount reclassified from OCI to profit or loss US$m

$ 2

Net operating costs [Member]

Disclosure Of Financial Instruments [Line Items]

Line item in the statement of profit or loss

Net operating costs (raw materials, consumables, repairs and maintenance)

Consolidated sales revenue [Member]

Disclosure Of Financial Instruments [Line Items]

Line item in the statement of profit or loss

Consolidated sales revenue