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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, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure Of Financial Instruments [Line Items]      
Total borrowings excluding overdrafts $ (12,752) $ (15,176)  
Cash and cash equivalents 10,773 10,550  
Net cash/(debt) 255 (3,845) $ (9,587)
Foreign exchange risk [member]      
Disclosure Of Financial Instruments [Line Items]      
Total borrowings excluding overdrafts (12,751)    
Derivatives related to net debt: designated as hedges (288)    
Cash and cash equivalents 10,772    
Other investments 2,522    
Net cash/(debt) 255 (3,845)  
Foreign exchange risk [member] | US Dollars [Member]      
Disclosure Of Financial Instruments [Line Items]      
Total borrowings excluding overdrafts (12,080)    
Derivatives related to net debt: designated as hedges (288)    
Cash and cash equivalents 10,310    
Other investments 2,522    
Net cash/(debt) 464 (3,628)  
Foreign exchange risk [member] | Australian dollar [Member]      
Disclosure Of Financial Instruments [Line Items]      
Total borrowings excluding overdrafts (472)    
Cash and cash equivalents 217    
Net cash/(debt) (255) (208)  
Foreign exchange risk [member] | Euro [Member]      
Disclosure Of Financial Instruments [Line Items]      
Total borrowings excluding overdrafts (39)    
Cash and cash equivalents 17    
Net cash/(debt) (22) (58)  
Foreign exchange risk [member] | South African rand [Member]      
Disclosure Of Financial Instruments [Line Items]      
Cash and cash equivalents 95    
Net cash/(debt) 95 59  
Foreign exchange risk [member] | Canadian dollar [Member]      
Disclosure Of Financial Instruments [Line Items]      
Total borrowings excluding overdrafts (157)    
Cash and cash equivalents 23    
Net cash/(debt) (134) (148)  
Foreign exchange risk [member] | Other currencies [member]      
Disclosure Of Financial Instruments [Line Items]      
Total borrowings excluding overdrafts (3)    
Cash and cash equivalents 110    
Net cash/(debt) $ 107 $ 138