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Purchases and sales of subsidiaries, joint ventures, associates and other interests in businesses - Additional information (Detail) - USD ($)
shares in Millions
 1 Months Ended 12 Months Ended
Sep. 01, 2017
Nov. 23, 2016
Jul. 20, 2016
Mar. 01, 2016
Jun. 17, 2015
Apr. 23, 2015
May 31, 2017
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Disclosure of acquisitions and disposals [Line Items]                      
Purchased amount             $ 194,000,000        
2017 disposals [member] | Yancoal Australia Limited [member]                      
Disclosure of acquisitions and disposals [Line Items]                      
Ownership interest in joint venture disposed 100.00%                    
Disposal amount $ 2,690,000,000                    
Cash consideration for disposal 2,450,000,000                 $ 2,540,000,000  
Unconditional guaranteed royalty payments received                   110,000,000  
Expected unconditional royalty payments to be received $ 240,000,000                 $ 130,000,000  
2016 disposals [member] | New hope corporation limited [member]                      
Disclosure of acquisitions and disposals [Line Items]                      
Ownership interest in joint venture disposed       40.00%              
Disposal amount       $ 616,700,000              
2016 disposals [member] | SIMEC [member]                      
Disclosure of acquisitions and disposals [Line Items]                      
Disposal amount   $ 410,000,000                  
Cash received                     $ 224,000,000
2015 disposals [member] | RZ Murowa Holdings Limited [member]                      
Disclosure of acquisitions and disposals [Line Items]                      
Ownership interest in cash generating units disposed         77.80%            
Proportion of ownership interest disposed         50.00%            
Simfer Jersey Limited [member]                      
Disclosure of acquisitions and disposals [Line Items]                      
Percentage of subsidiary             53.00%        
Percentage of purchased interest             53.00%        
Ownership interest                   53.00%  
Simfer S.A. [member]                      
Disclosure of acquisitions and disposals [Line Items]                      
Percentage of subsidiary             4.25%        
Percentage of purchased interest             4.25%        
Non-controlling interest (%), Subsidiary                   45.05%  
Ownership interest                   85.00%  
Simfer S.A. [member] | 2016 disposals [member]                      
Disclosure of acquisitions and disposals [Line Items]                      
Percentage of non-contributory shares     7.50%                
Percentage of ordinary contributory shares upon second tranche     10.00%                
Percentage of ordinary contributory shares     5.00%                
Simfer S.A. [member] | Bottom of range [Member]                      
Disclosure of acquisitions and disposals [Line Items]                      
Percentage of subsidiary             42.80%        
Percentage of purchased interest             42.80%        
Simfer S.A. [member] | Top of range [Member]                      
Disclosure of acquisitions and disposals [Line Items]                      
Percentage of subsidiary             45.05%        
Percentage of purchased interest             45.05%        
Carbone savoie [Member] | 2016 disposals [member] | Alandia industries [member]                      
Disclosure of acquisitions and disposals [Line Items]                      
Ownership interest in cash generating units disposed                 100.00%    
Bougainville Copper Limited [Member]                      
Disclosure of acquisitions and disposals [Line Items]                      
Non-controlling interest (%), Subsidiary                     36.40%
Bougainville Copper Limited [Member] | Equity Trustees Limited [Member]                      
Disclosure of acquisitions and disposals [Line Items]                      
Disposal amount               $ 0      
Percentage of share holding transferred               53.83%      
Bougainville Copper Limited [Member] | Autonomous Bougainville Government [Member]                      
Disclosure of acquisitions and disposals [Line Items]                      
Percentage of ownership shares distributed                     36.40%
Bougainville Copper Limited [Member] | Independent State of Papua New Guinea [Member]                      
Disclosure of acquisitions and disposals [Line Items]                      
Percentage of ownership shares distributed                     17.40%
Lochaber [member] | 2016 disposals [member] | SIMEC [member]                      
Disclosure of acquisitions and disposals [Line Items]                      
Proportion of ownership interest disposed   100.00%                  
Turquoise Hill Resources Ltd [member]                      
Disclosure of acquisitions and disposals [Line Items]                      
Ownership interest                   50.79%  
Turquoise Hill Resources Ltd [member] | 2015 disposals [member]                      
Disclosure of acquisitions and disposals [Line Items]                      
Sale of common shares           48.7          
Turquoise Hill Resources Ltd [member] | Top of range [Member] | 2015 disposals [member]                      
Disclosure of acquisitions and disposals [Line Items]                      
Ownership interest           20.00%