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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
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