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Capitalised finance leases - Summary of capitalised finance leases (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Disclosure Of Maturity Analysis Of Finance Lease Payments Payable [Line Items]    
Total minimum lease payments undiscounted $ 57 $ 45
Effect of discounting (4)  
Total minimum lease payments discounted 53 45
Due within 1 year [member]    
Disclosure Of Maturity Analysis Of Finance Lease Payments Payable [Line Items]    
Present value of minimum lease payments 4 21
Between 1 and 3 years [member]    
Disclosure Of Maturity Analysis Of Finance Lease Payments Payable [Line Items]    
Present value of minimum lease payments 14 8
Between 3 and 5 years [member]    
Disclosure Of Maturity Analysis Of Finance Lease Payments Payable [Line Items]    
Present value of minimum lease payments 28 6
More than 5 years [member]    
Disclosure Of Maturity Analysis Of Finance Lease Payments Payable [Line Items]    
Present value of minimum lease payments $ 11 $ 10