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Operating segments - Summary of non-current assets other than excluded items (Parenthetical) (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Disclosure Of Geographical Areas [Line Items]    
Non-current non-quasi equity loans to equity accounted units $ 39 $ 39
Quasi equity loans [member]    
Disclosure Of Geographical Areas [Line Items]    
Quasi equity loans to equity accounted units 159 163
Non-quasi equity loan [member]    
Disclosure Of Geographical Areas [Line Items]    
Non-current non-quasi equity loans to equity accounted units 39 39
Non-current assets [member]    
Disclosure Of Geographical Areas [Line Items]    
Investments in equity accounted units $ 4,327 $ 4,856