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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of operating segments [Line Items]        
Profit before taxation   $ 11,119 $ 18,167 $ 12,816
Profit after tax for the year   6,972 13,925 8,851
Post-tax foreign exchange gains   60 936  
Post-tax foreign exchange loss   9 $ 386  
US Dollar Net Debt [Member]        
Disclosure of operating segments [Line Items]        
Post-tax foreign exchange gains       420
Intragroup balances [Member]        
Disclosure of operating segments [Line Items]        
Post-tax foreign exchange loss       $ 908
Rossing Uranium Limited [Member]        
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 Llc [Member]        
Disclosure of operating segments [Line Items]        
Write-back of net realisable value provision   75    
Impairment gain (loss) excluded from underlying earnings   19    
Obligations On Legacy Operations [Member]        
Disclosure of operating segments [Line Items]        
Write-back of net realisable value provision   246    
Impairment gain (loss) excluded from underlying earnings   $ (233)