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Financial instruments and risk management - Summary of maximum credit risk exposure of the Group's financial assets (Detail) - Credit risk [Member] - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Disclosure Of Credit Risk Exposure [Line Items]    
Maximum credit risk exposure $ 14,993 $ 12,273
Cash and cash equivalents [member]    
Disclosure Of Credit Risk Exposure [Line Items]    
Maximum credit risk exposure 10,550 8,201
Trade and other receivables [member]    
Disclosure Of Credit Risk Exposure [Line Items]    
Maximum credit risk exposure 2,985 3,047
Derivative assets [member]    
Disclosure Of Credit Risk Exposure [Line Items]    
Maximum credit risk exposure 267 532
Loans to equity accounted units [member]    
Disclosure Of Credit Risk Exposure [Line Items]    
Maximum credit risk exposure 39 39
Fair value [member] | Loans And Receivables [Member] | Held to maturity assets [member] | Investments [member]    
Disclosure Of Credit Risk Exposure [Line Items]    
Maximum credit risk exposure $ 1,152 $ 454