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Intangible assets - Summary of intangible assets (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets at beginning of period $ 3,279 $ 3,336  
Adjustment on currency translation 216 58  
Expenditure during the year 122 31  
Amortisation for the year (177) (227) $ (207)
Impairment charges (357) (1)  
Disposals, transfers and other movements 36 82  
Intangible assets at end of period 3,119 3,279 3,336
Cost [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets at beginning of period 8,013    
Intangible assets at end of period 7,857 8,013  
Accumulated amortisation and impairment [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets at beginning of period (4,734)    
Intangible assets at end of period (4,738) (4,734)  
Exploration and evaluation [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets at beginning of period 711 706  
Adjustment on currency translation 27 14  
Expenditure during the year 57 8  
Impairment charges (357)    
Disposals, transfers and other movements (45) (17)  
Intangible assets at end of period 393 711 706
Exploration and evaluation [member] | Cost [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets at beginning of period 2,588    
Intangible assets at end of period 2,658 2,588  
Exploration and evaluation [member] | Accumulated amortisation and impairment [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets at beginning of period (1,877)    
Intangible assets at end of period (2,265) (1,877)  
Trademarks, patented and non-patented technology [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets at beginning of period 78 94  
Adjustment on currency translation 10 (3)  
Amortisation for the year (13) (13)  
Intangible assets at end of period 75 78 94
Trademarks, patented and non-patented technology [member] | Cost [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets at beginning of period 199    
Intangible assets at end of period 224 199  
Trademarks, patented and non-patented technology [member] | Accumulated amortisation and impairment [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets at beginning of period (121)    
Intangible assets at end of period (149) (121)  
Contract based intangible assets [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets at beginning of period 2,103 2,138  
Adjustment on currency translation 150 47  
Amortisation for the year (67) (87)  
Disposals, transfers and other movements 2 5  
Intangible assets at end of period 2,188 2,103 2,138
Contract based intangible assets [member] | Cost [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets at beginning of period 3,897    
Intangible assets at end of period 3,438 3,897  
Contract based intangible assets [member] | Accumulated amortisation and impairment [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets at beginning of period (1,794)    
Intangible assets at end of period (1,250) (1,794)  
Other intangible assets [Member]      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets at beginning of period 387 398  
Adjustment on currency translation 29    
Expenditure during the year 65 23  
Amortisation for the year (97) (127)  
Impairment charges   (1)  
Disposals, transfers and other movements 79 94  
Intangible assets at end of period 463 387 $ 398
Other intangible assets [Member] | Cost [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets at beginning of period 1,329    
Intangible assets at end of period 1,537 1,329  
Other intangible assets [Member] | Accumulated amortisation and impairment [member]      
Disclosure of detailed information about intangible assets [Line Items]      
Intangible assets at beginning of period (942)    
Intangible assets at end of period $ (1,074) $ (942)