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Financial instruments and risk management - Summary of derivative financial instruments (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure Of Financial Instruments [Line Items]    
Total net derivative instruments $ (264) $ 9
Derivative financial instruments 267 532
Non-current derivative financial instruments 238 508
Current derivative financial instruments 29 24
– current liabilities (50) (6)
– non-current liabilities (481) (517)
Total net derivatives instruments, detailed above (264) 9
Fair value hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial instruments 267 532
Derivative financial liabilities (531) (523)
Derivatives designated as hedges [member] | Fair value hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial instruments 99 109
Derivative financial liabilities (276) (516)
Derivatives not designated as hedges [member] | Fair value hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial instruments 168 423
Derivative financial liabilities (255) (7)
Interest rate swaps [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial instruments 99 108
Interest rate swaps [member] | Derivatives designated as hedges [member] | Fair value hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial instruments 99 108
Derivative financial liabilities (127) (134)
Cross currency interest rate swaps [member] | Derivatives designated as hedges [member] | Fair value hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial liabilities (149) (382)
Currency forward contracts, options and swaps [member] | Derivatives designated as hedges [member] | Fair value hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial instruments   1
Currency forward contracts, options and swaps [member] | Derivatives not designated as hedges [member] | Fair value hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial instruments 7 1
Derivative financial liabilities (2)  
Aluminium forward contracts [member] | Derivatives not designated as hedges [member] | Fair value hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial instruments 21 6
Derivative financial liabilities (9) (2)
Aluminium embedded derivatives [member] | Derivatives not designated as hedges [member] | Fair value hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial instruments 140 411
Derivative financial liabilities (238) (3)
Other embedded derivatives [member] | Derivatives not designated as hedges [member] | Fair value hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial instruments   5
Derivative financial liabilities (5)  
Other commodity contracts [member] | Derivatives not designated as hedges [member] | Fair value hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial liabilities (1) (2)
Due within 1 year [member] | Fair value hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial instruments 29 24
Derivative financial liabilities (50) (6)
Pre 2021 [Member] | Fair value hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial instruments 31 110
Derivative financial liabilities (233) (194)
More than 5 years [member] | Fair value hedges [member]    
Disclosure Of Financial Instruments [Line Items]    
Derivative financial instruments 207 398
Derivative financial liabilities $ (248) $ (323)