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Principal accounting policies - Additional information (Detail)
 12 Months Ended
Dec. 31, 2017
USD ($)
Segment
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Disclosure of initial application of standards or interpretations [Line Items]      
Derecognise a receivable $ 235,000,000    
Derecognise a receivable after tax 164,000,000    
Consolidated sales revenue 40,030,000,000 $ 33,781,000,000 $ 34,829,000,000
Investment on money market fund $ 7,800,000,000    
Voting rights of associate to have significant infuluence 20.00%    
Cash-generating units assessed for impairment 0    
Onerous contracts provision $ 407,000,000    
Operations with remaining lives 10 years    
Number of operations | Segment 10    
Decreasing provision $ 700,000,000    
Provision for carrying value close-down, restoration and environmental $ 9,983,000,000 $ 8,722,000,000  
Assumption of discount rates 0.50%    
Risk free interest rate on provisions 2.00%    
Intragroup loans $ 21,000,000,000    
Bottom of range [Member]      
Disclosure of initial application of standards or interpretations [Line Items]      
Percent of certainty 50.00%    
Assessment period of market prices three years    
Term of impairment review is under taken in relevant functional currency five years    
Risk free interest rate on provisions 0.50%    
Top of range [Member]      
Disclosure of initial application of standards or interpretations [Line Items]      
Percent of certainty 90.00%    
Assessment period of market prices five years    
Increase (decrease) due to voluntary changes in accounting policy [member]      
Disclosure of initial application of standards or interpretations [Line Items]      
Consolidated sales revenue $ (15,000,000)