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Deferred Taxation - Recognised and unrecognised amounts in deferred tax assets (Footnotes) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Deferred tax assets $ 3,385 $ 3,102
Unrecognised deferred tax assets 3,841 3,162
Realised or unrealised capital losses    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Deferred tax assets $ 551 491
Deferred tax loss recovery period 1 year  
Unrecognised deferred tax assets $ 720 695
Mongolia    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Deferred tax assets 974 704
Unrecognised deferred tax assets 540 721
Mongolia | Tax losses    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Deferred tax assets $ 292 $ 130
Deferred tax loss recovery period 8 years