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Finance income and finance costs - Summary of finance income and finance costs (Parenthetical) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of borrowings and other financial liabilities [Line Items]      
Buy-back of bonds     $ 1,900
Premium charges for redemption of bonds     72
Unamortized debt issuance costs and fees     9
Fair value hedge adjustment     16
Net interest paid associated with bond redepemption     80
Cost of hedging reserve      
Disclosure of borrowings and other financial liabilities [Line Items]      
Transfer of cost of hedging to the income statement $ 0 $ 0 $ (3)