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Impairment charges - Narrative (Details) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Jul. 09, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Disclosure of impairment loss recognised or reversed for cash-generating unit [line items]        
Impairment charges, net   $ 0 $ (1,033)  
Impairment loss   0 1,134  
Impairment loss of equity accounted unit   0 148  
Investment in equity accounted units ('EAUs')        
Disclosure of impairment loss recognised or reversed for cash-generating unit [line items]        
Impairment loss   0 119  
Pacific Aluminum        
Disclosure of impairment loss recognised or reversed for cash-generating unit [line items]        
Contract termination notice period 14 months      
Pacific Aluminum | Aluminium, Alumina and Bauxite        
Disclosure of impairment loss recognised or reversed for cash-generating unit [line items]        
Recoverable amount of asset or cash-generating unit   $ 273    
Discount rate applied to cash flow projections   6.60%    
Impairment loss   $ 0 489  
Impairment loss     26  
Pacific Aluminum | Aluminium, Alumina and Bauxite | Pre-tax        
Disclosure of impairment loss recognised or reversed for cash-generating unit [line items]        
Impairment loss     36  
ISAL | Aluminium, Alumina and Bauxite        
Disclosure of impairment loss recognised or reversed for cash-generating unit [line items]        
Recoverable amount of asset or cash-generating unit     $ 139  
Discount rate applied to cash flow projections     6.60%  
Impairment loss   0 $ 204  
Impairment loss     204 $ 93
Reversal of impairment loss recognised in profit or loss     111  
Diavik | Minerals        
Disclosure of impairment loss recognised or reversed for cash-generating unit [line items]        
Impairment loss   $ 0 $ 441  
Proportion of ownership interest in joint venture (as a percent)   40.00%