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Principal accounting policies - Additional information (Detail)
 12 Months Ended
Dec. 31, 2018
USD ($)
Operation
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Disclosure of initial application of standards or interpretations [Line Items]      
Retained earnings $ 27,025,000,000 $ 23,761,000,000  
Weighted average incremental borrowing rate applied to lease liabilities 4.70%    
Undiscounted non-cancellable operating lease commitments $ 1,717,000,000 1,845,000,000  
Non-cancellable operating lease commitments 500,000    
Non-cancellable operating lease commitments not expected to be charged against EBITDA 300,000    
Operating lease expenses $ 787,000,000 555,000,000 $ 541,000,000
Cash-generating units assessed for impairment 0    
Depreciation or amortisation charges $ 4,015,000,000 4,375,000,000 $ 4,794,000,000
Onerous contracts provision $ 324,000,000    
Operations with remaining lives 10 years    
Number of operations | Operation 13    
Decreasing provision $ 800,000,000    
Provision for carrying value close-down, restoration and environmental $ 9,975,000,000 $ 9,983,000,000  
Risk free rate decrease 0.50%    
Risk free interest rate on provisions 2.00%    
Intragroup loans $ 14,000,000,000    
Intangible assets [Member]      
Disclosure of initial application of standards or interpretations [Line Items]      
Depreciation or amortisation charges $ 0    
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    
IFRS 16 [Member]      
Disclosure of initial application of standards or interpretations [Line Items]      
Increase in lease liabilities (debt) $ 1,200,000,000    
Increase in right of use assets/net investments in leases 1,100,000,000    
Net adjustments to other assets and liabilities 100,000,000    
Retained earnings 100,000,000    
Lease liability 1,300,000,000    
IAS 17 [Member]      
Disclosure of initial application of standards or interpretations [Line Items]      
Undiscounted non-cancellable operating lease commitments 1,700,000,000    
IAS 17 [Member] | Entering into significant commitments or contingent liabilities      
Disclosure of initial application of standards or interpretations [Line Items]      
Undiscounted non-cancellable operating lease commitments $ 1,700,000,000