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Investments in equity accounted units - Additional information (Detail) - USD ($)
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
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 (664,000,000) (3,651,000,000) $ 255,000,000
Investment in equity accounted units      
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 $ (931,000,000) $ (1,248,000,000)