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Financial instruments and risk management - Summary of changes in fair value of Level 3 financial assets and financial liabilities (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure Of Financial Instruments [Line Items]      
– Consolidated sales revenue $ (40,030) $ (33,781) $ (34,829)
– Net operating costs 26,983 26,799 27,919
Level 3 [member]      
Disclosure Of Financial Instruments [Line Items]      
Financial assets and liabilities at beginning balance 479 456  
Currency translation adjustments 8 (2)  
– Consolidated sales revenue 1 1  
– Net operating costs (5) (28)  
- Consolidated sales revenue 17    
– Net operating costs (508) 11  
Additions   43  
Impairment   (2)  
Disposals/maturity of financial instruments (5)    
Transfers 6    
Financial assets and liabilities at ending balance (7) 479 $ 456
Total (losses)/gains for the year included in the income statement for assets and liabilities held at year end $ (491) $ 11