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Financial instruments and risk management - Summary by currency, Group's net debt, after taking into account relevant cross currency interest rate swaps and foreign exchange contracts (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure Of Financial Instruments [Line Items]      
Cash and cash equivalents $ 10,550 $ 8,201  
Total borrowings in note 22 (15,176) (17,630)  
Derivatives related to net debt (276) (516)  
Net funds/(debt) (3,845) (9,587) $ (13,783)
Foreign exchange risk [member]      
Disclosure Of Financial Instruments [Line Items]      
Cash and cash equivalents 10,550    
Other investments 958    
Total borrowings in note 22 (15,176)    
Derivatives related to net debt (177)    
Net funds/(debt) (3,845) (9,587)  
Foreign exchange risk [member] | US Dollars [Member]      
Disclosure Of Financial Instruments [Line Items]      
Cash and cash equivalents 9,950    
Other investments 958    
Total borrowings in note 22 (14,359)    
Derivatives related to net debt (177)    
Net funds/(debt) (3,628) (9,294)  
Foreign exchange risk [member] | Australian Dollars [Member]      
Disclosure Of Financial Instruments [Line Items]      
Cash and cash equivalents 332    
Total borrowings in note 22 (540)    
Net funds/(debt) (208) (313)  
Foreign exchange risk [member] | Euro Member Countries, Euro      
Disclosure Of Financial Instruments [Line Items]      
Cash and cash equivalents 49    
Total borrowings in note 22 (107)    
Net funds/(debt) (58) (62)  
Foreign exchange risk [member] | South Africa, Rand      
Disclosure Of Financial Instruments [Line Items]      
Cash and cash equivalents 59    
Net funds/(debt) 59 103  
Foreign exchange risk [member] | Canada, Dollars [Member]      
Disclosure Of Financial Instruments [Line Items]      
Cash and cash equivalents 18    
Total borrowings in note 22 (166)    
Net funds/(debt) (148) (155)  
Foreign exchange risk [member] | Other Currencies [member]      
Disclosure Of Financial Instruments [Line Items]      
Cash and cash equivalents 142    
Total borrowings in note 22 (4)    
Net funds/(debt) $ 138 $ 134