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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
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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 |  |

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