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Investments in equity accounted units - Additional information (Detail) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of Investments in equity accounted units [Line Items]      
Investments in equity accounted units with shares listed on recognised stock exchanges $ 0 $ 0  
Net debt of the Group 3,845,000,000 9,587,000,000 $ 13,783,000,000
Investment in equity accounted units [member]      
Disclosure of Investments in equity accounted units [Line Items]      
Net debt of the Group $ 1,244,000,000 $ 1,338,000,000