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Deferred Taxation - Recognised and unrecognised amounts in deferred tax assets (Parenthetical) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Deferred tax assets $ 3,395 $ 3,728
Deferred tax loss recovery period 6 years  
Unrecognised deferred tax assets $ 3,676 3,442
Realised or unrealised capital losses [member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Deferred tax assets $ 250 105
Deferred tax loss recovery period 1 year  
Unrecognised deferred tax assets $ 777 964
Mongolia [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Deferred tax assets 631 441
Unrecognised deferred tax assets 61  
Mongolia [Member] | Tax losses [member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Deferred tax assets $ 432 $ 365
Deferred tax loss recovery period 8 years