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Financial instruments and risk management - Summary of derivative financial instruments (Detail) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Disclosure of detailed information about financial instruments [line items]    
Derivative financial assets $ 366 $ 556
Derivative financial liabilities (351) (502)
Disclosure Of Information About Amounts Recognised In Balance Sheet [Abstract]    
Non-current assets 308 468
Current assets 58 88
Current liabilities (103) (95)
Non-current liabilities (248) (407)
Total net derivative instruments 15 54
Within 1 year [member]    
Disclosure of detailed information about financial instruments [line items]    
Derivative financial assets 58 88
Derivative financial liabilities (103) (95)
Between 1 and 5 years [member]    
Disclosure of detailed information about financial instruments [line items]    
Derivative financial assets 96 153
Derivative financial liabilities (86) (205)
More than 5 years [member]    
Disclosure of detailed information about financial instruments [line items]    
Derivative financial assets 212 315
Derivative financial liabilities (162) (202)
Other Financial Assets    
Disclosure of detailed information about financial instruments [line items]    
Total net derivative instruments 15 54
Derivatives designated as hedges    
Disclosure of detailed information about financial instruments [line items]    
Derivative financial assets 217 124
Derivative financial liabilities (322) (404)
Derivatives designated as hedges | Interest rate swaps [Member]    
Disclosure of detailed information about financial instruments [line items]    
Derivative financial assets 151 70
Derivative financial liabilities (38) (137)
Derivatives designated as hedges | Cross currency interest rate swaps    
Disclosure of detailed information about financial instruments [line items]    
Derivative financial assets 0 0
Derivative financial liabilities (260) (221)
Derivatives designated as hedges | Aluminium Embedded Derivatives    
Disclosure of detailed information about financial instruments [line items]    
Derivative financial assets 66 54
Derivative financial liabilities (24) (46)
Derivatives not designated as hedges    
Disclosure of detailed information about financial instruments [line items]    
Derivative financial assets 149 432
Derivative financial liabilities (29) (98)
Derivatives not designated as hedges | Aluminium Embedded Derivatives    
Disclosure of detailed information about financial instruments [line items]    
Derivative financial assets 96 346
Derivative financial liabilities 0 0
Derivatives not designated as hedges | Currency Forward Contracts Options And Swaps    
Disclosure of detailed information about financial instruments [line items]    
Derivative financial assets 13 0
Derivative financial liabilities (5) (68)
Derivatives not designated as hedges | Other embedded derivatives    
Disclosure of detailed information about financial instruments [line items]    
Derivative financial assets 14 6
Derivative financial liabilities 0 0
Derivatives not designated as hedges | Other commodity contracts    
Disclosure of detailed information about financial instruments [line items]    
Derivative financial assets 26 80
Derivative financial liabilities $ (24) $ (30)