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Property, plant and equipment Property, plant, and equipment - Summary of Right-of-Use Assets (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of detailed information about property, plant and equipment [Line Items]      
Net book value $ 875 $ 1,065  
Additions for the year 105 301  
Depreciation for the year (included within operating costs) (322) (272)  
Impairment charges (904) (3,487) $ (132)
Land and buildings      
Disclosure of detailed information about property, plant and equipment [Line Items]      
Net book value 475 507  
Additions for the year 30 89  
Depreciation for the year (included within operating costs) (93) (69)  
Plant and equipment      
Disclosure of detailed information about property, plant and equipment [Line Items]      
Net book value 400 558  
Additions for the year 75 212  
Depreciation for the year (included within operating costs) (229) (203)  
Right-of-use assets      
Disclosure of detailed information about property, plant and equipment [Line Items]      
Impairment charges (37) (10)  
Right-of-use assets | Land and buildings      
Disclosure of detailed information about property, plant and equipment [Line Items]      
Impairment charges (6) (6)  
Right-of-use assets | Plant and equipment      
Disclosure of detailed information about property, plant and equipment [Line Items]      
Impairment charges $ (31) $ (4)