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Deferred Taxation - Recognised and unrecognised amounts in deferred tax assets (Footnotes) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Deferred tax assets $ 3,102 $ 3,137 $ 3,137
Unrecognised deferred tax assets 3,162 2,510 $ 2,510
Realised or unrealised capital losses [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Deferred tax assets $ 491 96  
Deferred tax loss recovery period 3 years    
Unrecognised deferred tax assets $ 695 684  
Mongolia [Member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Deferred tax assets 704 703  
Unrecognised deferred tax assets 721 87  
Mongolia [Member] | Tax losses [Member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Deferred tax assets $ 130 $ 469  
Deferred tax loss recovery period 8 years