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Acquisitions and disposals - Additional information (Detail) - USD ($)
 1 Months Ended 6 Months Ended
May 10, 2018
Mar. 27, 2018
Mar. 22, 2018
Mar. 20, 2018
Jan. 24, 2017
May 31, 2017
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2019
Feb. 26, 2018
Jan. 10, 2018
Disclosure Of Acquisitions And Disposals [Line Items]                      
Disposal amount               $ 0      
Disposals of subsidiaries, joint ventures and associates [1]             $ 402,000,000 135,000,000      
Purchase of non-controlling interests [2]               $ 194,000,000      
Simfer Jersey Limited [member]                      
Disclosure Of Acquisitions And Disposals [Line Items]                      
Ownership percentage in subsidiary           53.00% 53.00%        
Simfer S.A. [member]                      
Disclosure Of Acquisitions And Disposals [Line Items]                      
Ownership percentage in subsidiary             85.00%        
Proportion of ownership interest in subsidiary purchased           4.25%          
Purchase of non-controlling interests           $ 194,000,000          
Non-controlling interest (%), Subsidiary             45.05%        
Simfer S.A. [member] | Bottom of range [Member]                      
Disclosure Of Acquisitions And Disposals [Line Items]                      
Non-controlling interest (%), Subsidiary           42.80%          
Simfer S.A. [member] | Top Of Range [Member]                      
Disclosure Of Acquisitions And Disposals [Line Items]                      
Non-controlling interest (%), Subsidiary           45.05%          
Group's Aluminium Assets Held for Sale [Member] | Dunkerque Aluminium Smelter [Member]                      
Disclosure Of Acquisitions And Disposals [Line Items]                      
Business acquisition binding offer amount                     $ 500,000,000
Group's Aluminium Assets Held for Sale [Member] | ISAL Smelter, Aluchemie Anode Plant and Aluminium Fluoride Plant [Member]                      
Disclosure Of Acquisitions And Disposals [Line Items]                      
Business acquisition binding offer amount                   $ 345,000,000  
Group's Aluminium Assets Held for Sale [Member] | Aluchemie Anode Plant [Member] | NETHERLANDS                      
Disclosure Of Acquisitions And Disposals [Line Items]                      
Percentage of ownership interest sold                   53.30%  
Group's Aluminium Assets Held for Sale [Member] | Aluminium Fluoride Plant [Member] | SWEDEN                      
Disclosure Of Acquisitions And Disposals [Line Items]                      
Percentage of ownership interest sold                   50.00%  
2018 Disposals [Member]                      
Disclosure Of Acquisitions And Disposals [Line Items]                      
Disposal amount       $ 1,700,000,000              
Elysis [Member]                      
Disclosure Of Acquisitions And Disposals [Line Items]                      
Cash investment in joint venture $ 5,000,000                    
Whitehaven Coal Limited [Member] | 2018 Disposals [Member]                      
Disclosure Of Acquisitions And Disposals [Line Items]                      
Disposal amount     $ 200,000,000                
Ownership interest in joint venture disposed     75.00%                
Disposals of subsidiaries, joint ventures and associates     $ 150,000,000                
Whitehaven Coal Limited [Member] | 2018 Disposals [Member] | Due in June 2019 [Member]                      
Disclosure Of Acquisitions And Disposals [Line Items]                      
Unconditional cash payment receivable from disposals                 $ 50,000,000    
Hail Creek Coal Mine [Member]                      
Disclosure Of Acquisitions And Disposals [Line Items]                      
Proportion of ownership interest in joint venture             82.00%        
Hail Creek Coal Mine [Member] | 2018 Disposals [Member]                      
Disclosure Of Acquisitions And Disposals [Line Items]                      
Ownership interest in joint venture disposed       82.00%              
Valeria Project [Member] | 2018 Disposals [Member]                      
Disclosure Of Acquisitions And Disposals [Line Items]                      
Ownership interest in joint venture disposed       71.20%              
Kestrel Underground Coal Mine [Member]                      
Disclosure Of Acquisitions And Disposals [Line Items]                      
Proportion of ownership interest in joint venture             80.00%        
Kestrel Underground Coal Mine [Member] | 2018 Disposals [Member]                      
Disclosure Of Acquisitions And Disposals [Line Items]                      
Disposal amount   $ 2,250,000,000                  
Proportion of ownership interest in joint venture   80.00%                  
Receipts from non-refundable deposits             $ 338,000,000        
Coal & Allied Industries Limited [member] | 2017 Disposals [Member]                      
Disclosure Of Acquisitions And Disposals [Line Items]                      
Disposal amount         $ 2,690,000,000            
Proportion of ownership interest in subsidiaries disposed         100.00%            
[1] Disposal proceeds in the six months to 30 June 2018 include cash proceeds of US$60 million received in respect of the non-contingent royalty receivable from Yancoal Australia Limited on the sale of Coal & Allied Industries Limited, which completed in the second half of 2017. In addition, non-refundable deposits of US$338 million have been received during the period relating to the agreed sale of the Kestrel underground coal mine. The sale completed on 1 August 2018.
[2] In May 2017, the Group’s subsidiary Simfer Jersey Limited (Rio Tinto 53%) purchased a 4.25% interest in Simfer SA from International Finance Corporation for US$194 million.