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Deferred Taxation - Reconciliation of changes in deferred tax liability asset (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation Of Changes In Deferred Tax Liability Asset [Abstract]      
At 1 January - deferred tax asset $ (607) $ (23)  
Adjustment on currency translation 53 58  
Charged/(credited) to the income statement 695 (548)  
(Charged)/credited to statement of comprehensive income 153 (33) $ 178
Disposals   (8)  
Other movements (61) (53)  
At 31 December - deferred tax liability/(asset) 233 (607) $ (23)
– deferred tax liabilities 3,628 3,121  
– deferred tax assets $ (3,395) $ (3,728)