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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, 2023
Dec. 31, 2022
Dec. 31, 2021
 [1]
Disclosure of detailed information about financial instruments [line items]      
Nominal $ 1,050 $ 1,216  
Losses reclassified from reserves to income statement 655 (2,357) [1] $ (2,030)
Aluminium embedded derivatives separated from the power contract      
Disclosure of detailed information about financial instruments [line items]      
Nominal 1,050 1,216  
Carrying amount (174) (189)  
Change in fair value in the period 3 (119)  
Embedded derivatives in other financial liabilities 174 189  
Highly probable forecast aluminum sales      
Disclosure of detailed information about financial instruments [line items]      
Cash flow hedge reserve (91) (87)  
Change in fair value in the period (16) 133  
Highly probable forecast aluminum sales | Cash flow hedge reserve      
Disclosure of detailed information about financial instruments [line items]      
Total hedging losses recognised in reserves (1) (110)  
Hedge ineffective-ness in the period gains/(losses) 4 (9)  
Losses reclassified from reserves to income statement $ (2) $ 34  
[1] Comparative information has been restated to reflect the adoption of narrow-scope amendments to IAS 12. Refer to page 166 for details.