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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, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of detailed information about financial instruments [line items]      
Total borrowings $ (13,831) $ (14,115)  
Cash and cash equivalents 10,381 8,027  
Other investments 2,899 2,839  
Net cash/(debt) (664) (3,651) $ 255
Foreign exchange risk      
Disclosure of detailed information about financial instruments [line items]      
Total borrowings (12,653)    
Lease liabilities (1,178)    
Derivatives related to net debt: designated as hedges 248    
Cash and cash equivalents 10,381    
Other investments 2,538    
Net cash/(debt) (664) (3,651)  
Foreign exchange risk | US dollar      
Disclosure of detailed information about financial instruments [line items]      
Total borrowings (12,102)    
Lease liabilities (342)    
Derivatives related to net debt: designated as hedges 248    
Cash and cash equivalents 9,517    
Other investments 2,538    
Net cash/(debt) (141) (2,843)  
Foreign exchange risk | Australian dollar      
Disclosure of detailed information about financial instruments [line items]      
Total borrowings (375)    
Lease liabilities (350)    
Derivatives related to net debt: designated as hedges 0    
Cash and cash equivalents 439    
Other investments 0    
Net cash/(debt) (286) (561)  
Foreign exchange risk | Euro      
Disclosure of detailed information about financial instruments [line items]      
Total borrowings (4)    
Lease liabilities (25)    
Derivatives related to net debt: designated as hedges 0    
Cash and cash equivalents 43    
Other investments 0    
Net cash/(debt) 14 10  
Foreign exchange risk | South African rand      
Disclosure of detailed information about financial instruments [line items]      
Total borrowings 0    
Lease liabilities (1)    
Derivatives related to net debt: designated as hedges 0    
Cash and cash equivalents 141    
Other investments 0    
Net cash/(debt) 140 171  
Foreign exchange risk | Canadian dollar      
Disclosure of detailed information about financial instruments [line items]      
Total borrowings (170)    
Lease liabilities (199)    
Derivatives related to net debt: designated as hedges 0    
Cash and cash equivalents 36    
Other investments 0    
Net cash/(debt) (333) (322)  
Foreign exchange risk | Other      
Disclosure of detailed information about financial instruments [line items]      
Total borrowings (2)    
Lease liabilities (261)    
Derivatives related to net debt: designated as hedges 0    
Cash and cash equivalents 205    
Other investments 0    
Net cash/(debt) $ (58) $ (106)