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Intangible assets - Summary of intangible assets (Footnotes) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets $ 2,755 $ 2,637 $ 2,779
Disposals, transfers and other movements $ 16 $ (170)  
Quebec smelters      
Disclosure of detailed information about intangible assets [Line Items]      
Fair value less costs of disposal, post-tax discount rate 6.60% 6.60%  
Water rights | Quebec smelters      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets $ 1,798 $ 1,759  
Trademarks | Bottom of range      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives 14    
Trademarks | Top of range      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives 20 years    
Patented and non-patented technology | Bottom of range      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives ten    
Patented and non-patented technology | Top of range      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives 20 years    
Power contracts water rights | Bottom of range      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives two    
Power contracts water rights | Top of range      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives 45 years    
Other purchase and customer contracts | Bottom of range      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives five    
Other purchase and customer contracts | Top of range      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives 15 years    
Computer software | Bottom of range      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives two    
Computer software | Top of range      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives five years    
Other intangible assets      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets $ 457 473 $ 505
Disposals, transfers and other movements $ 23 $ 47  
Other intangible assets | Bottom of range      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives two    
Other intangible assets | Top of range      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives 20 years