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Intangible assets - Summary of intangible assets (Footnotes) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets $ 2,637 $ 2,779 $ 3,119
Disposals, transfers and other movements $ (170) $ (134)  
Quebec smelters      
Disclosure of detailed information about intangible assets [Line Items]      
Fair value less costs of disposal, post-tax discount rate 6.60% 6.80%  
Water rights | Quebec smelters      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets $ 1,759 $ 1,684  
Trademarks | Bottom of range [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives P14Y    
Trademarks | Top of range [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives P20Y    
Patented and non-patented technology | Bottom of range [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives P10Y    
Patented and non-patented technology | Top of range [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives P20Y    
Power contracts water rights | Bottom of range [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives P2Y    
Power contracts water rights | Top of range [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives P45Y    
Other purchase and customer contracts | Bottom of range [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives P5Y    
Other purchase and customer contracts | Top of range [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives P15Y    
Computer software | Bottom of range [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives P2Y    
Computer software | Top of range [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives P5Y    
Other intangible assets      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets $ 473 505 463
Disposals, transfers and other movements $ 47 103  
Other intangible assets | Bottom of range [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives P2Y    
Other intangible assets | Top of range [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives P20Y    
Exploration and evaluation      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets $ 173 233 $ 393
Disposals, transfers and other movements $ (116) $ (225)