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Operating segments - Reconciliation of net earnings/(losses) to underlying earnings (Parenthetical) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Jul. 16, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of operating segments [Line Items]        
Profit before taxation   $ 15,391 $ 11,119 $ 18,167
Profit after tax for the year   10,400 6,972 13,925
Post-tax foreign exchange gains   1,330 9  
Post-tax foreign exchange loss   $ 205 60  
Closure discount rate   1.50%    
US Dollar Net Debt        
Disclosure of operating segments [Line Items]        
Post-tax foreign exchange gains       386
Intragroup balances        
Disclosure of operating segments [Line Items]        
Post-tax foreign exchange loss       $ 936
Rossing Uranium Limited        
Disclosure of operating segments [Line Items]        
Ownership percentage in subsidiary 68.62%      
Profit before taxation $ 289      
Profit after tax for the year $ 289      
Oyu Tolgoi        
Disclosure of operating segments [Line Items]        
Write-back of net realisable value provision     75  
Impairment gain (loss) excluded from underlying earnings     19  
Gove Refinery        
Disclosure of operating segments [Line Items]        
Ownership percentage in subsidiary   100.00%    
Profit before taxation   $ 138    
Obligations On Legacy Operations        
Disclosure of operating segments [Line Items]        
Write-back of net realisable value provision     246  
Impairment gain (loss) excluded from underlying earnings     $ (233)