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Events after the balance sheet date - Additional information (Detail) - USD ($)
$ in Millions
Feb. 26, 2018
Feb. 07, 2018
Jan. 10, 2018
Sep. 21, 2017
Disclosure Of Nonadjusting Events After Reporting Period [Line Items]        
On-market share buy-back amount   $ 1,000   $ 2,500
Stock buy-back program during period amount   1,925    
Group’s Aluminium Dunkerque Smelter In Northern France [Member}        
Disclosure Of Nonadjusting Events After Reporting Period [Line Items]        
Business acquisition binding offer amount $ 345      
Group’s Aluminium Dunkerque Smelter In Northern France [Member} | Aluminerie Alouette Inc. [member] | NETHERLANDS        
Disclosure Of Nonadjusting Events After Reporting Period [Line Items]        
Percentage of ownership interest sold 53.30%      
Group’s Aluminium Dunkerque Smelter In Northern France [Member} | Alufluor Ab | SWEDEN        
Disclosure Of Nonadjusting Events After Reporting Period [Line Items]        
Percentage of ownership interest sold 50.00%      
Group’s Aluminium Dunkerque Smelter In Northern France [Member} | Dunkerque Aluminium Smelter [Member]        
Disclosure Of Nonadjusting Events After Reporting Period [Line Items]        
Business acquisition binding offer amount     $ 500  
Share Buy-back Programme [Member]        
Disclosure Of Nonadjusting Events After Reporting Period [Line Items]        
On-market share buy-back amount   1,000    
Stock buy-back program during period amount   $ 1,925