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Intangible assets - Summary of intangible assets (Parenthetical) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets $ 2,779 $ 3,119 $ 3,279
Disposals, transfers and other movements $ (134) $ 36  
Quebec smelters [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Fair value less costs of disposal, post-tax discount rate 6.80% 6.70%  
Description of recoverable amount of asset is fair value less costs of disposal or value in use the Quebec smelters CGU. The Quebec smelters CGU was tested for impairment by reference to FVLCD using discounted cash flows, which is in line with the policy set out in note 1(i). The recoverable amount of the Quebec smelters is classified as level 3 under the fair value hierarchy    
Description of level of fair value hierarchy within which fair value measurement is categorized level 3    
Water rights [member] | Quebec smelters [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets $ 1,684    
Trademarks [member] | Bottom of range [Member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives 14 years    
Trademarks [member] | Top of range [Member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives 20 years    
Patented and non-patented technology [member] | Bottom of range [Member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives ten years    
Patented and non-patented technology [member] | Top of range [Member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives 20 years    
Power contracts water rights [member] | Bottom of range [Member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives two years    
Power contracts water rights [member] | Top of range [Member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives 45 years    
Other purchase and customer contracts [member] | Bottom of range [Member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives five years    
Other purchase and customer contracts [member] | Top of range [Member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives 15 years    
Computer software [Member] | Bottom of range [Member] | Internally generated [Member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives two years    
Computer software [Member] | Top of range [Member] | Internally generated [Member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives five years    
Other intangible assets [Member]      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets $ 505 $ 463 387
Disposals, transfers and other movements $ 103 79  
Other intangible assets [Member] | Bottom of range [Member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives two years    
Other intangible assets [Member] | Top of range [Member]      
Disclosure of detailed information about intangible assets [Line Items]      
Finite life intangible assets, useful lives 20 years    
Exploration and evaluation [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets $ 233 393 $ 711
Disposals, transfers and other movements $ (225) (45)  
Exploration and evaluation [member] | Kemano Tunnel Project [Member]      
Disclosure of detailed information about intangible assets [Line Items]      
Disposals, transfers and other movements   $ 34