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Financial instruments and risk management - Summary of derivative financial instruments (Detail) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Disclosure Of Financial Instruments [Line Items]    
Total net derivative instruments $ 54 $ (264)
Derivative financial assets 556 267
Derivative financial liabilities (502) (531)
Non-current derivative financial instruments 468 238
Current derivative financial instruments 88 29
Current liabilities (95) (50)
Non-current liabilities (407) (481)
Total net derivative instruments 54 (264)
Derivatives designated as hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial assets 124 99
Derivative financial liabilities (404) (276)
Derivatives not designated as hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial assets 432 168
Derivative financial liabilities (98) (255)
Interest rate swaps [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial assets 70 99
Interest rate swaps [member] | Derivatives designated as hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial assets 70 99
Derivative financial liabilities (137) (127)
Cross currency interest rate swaps [member] | Derivatives designated as hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial liabilities (221) (149)
Aluminium Embedded Derivatives | Derivatives designated as hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial assets 54  
Derivative financial liabilities (46)  
Aluminium Embedded Derivatives | Derivatives not designated as hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial assets 346 140
Derivative financial liabilities   (238)
Currency Forward Contracts Options And Swaps | Derivatives not designated as hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial assets   7
Derivative financial liabilities (68) (2)
Aluminium forward contracts [member] | Derivatives not designated as hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial assets 26 21
Derivative financial liabilities (19) (9)
Other embedded derivatives [member] | Derivatives not designated as hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial assets 6  
Derivative financial liabilities   (5)
Other commodity contracts [member] | Derivatives not designated as hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial assets 54  
Derivative financial liabilities (11) (1)
Due within 1 year [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial assets 88 29
Derivative financial liabilities (95) (50)
Between 1 and 5 years [Member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial assets 153 31
Derivative financial liabilities (205) (233)
More than 5 years [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial assets 315 207
Derivative financial liabilities $ (202) $ (248)