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Financial instruments and risk management - Interest rate risk - Additional information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Interest rate swaps [member] | US Dollars [Member]    
Disclosure Of Financial Instruments [Line Items]    
Notional amount $ 4,300,000,000 $ 5,700,000,000
Interest rate risk [Member]    
Disclosure Of Financial Instruments [Line Items]    
Adjusted total borrowings $ 10,200,000,000 $ 12,500,000,000
Weighted average maturity period 11 years ten years
Description of borrowings, interest rate basis the effect on net earnings of a 100 basis point increase in US dollar LIBOR interest rates, with all other variables held constant, would be an income of US$8 million (2017: charge of US$8 million) due to the net cash position in 2018 versus the net debt position in 2017. The Group has an exposure to interest rate volatility within shareholders’ equity arising from fair value movements on derivatives in the cash flow hedge reserve. These derivatives have an underlying exposure to sterling and US dollars. With all factors remaining constant, and based on the composition of derivatives impacting the cash flow reserve at 31 December 2018, the sensitivity of a 100 basis point increase in interest rates in each of the currencies in isolation would impact equity, before tax, by a charge of US$69 million (2017: US$84 million charge) for sterling and a credit of US$78 million (2017: US$88 million credit) for US dollars. A 100 basis point decrease would have broadly the same impact in the opposite direction. These balances will not remain constant throughout 2019, and therefore this information should be used with care.  
Interest rate risk [Member] | United Kingdom, Pounds    
Disclosure Of Financial Instruments [Line Items]    
Increase in financial derivative charges on interest rate before tax $ 69,000,000 $ 84,000,000
Interest rate risk [Member] | US Dollars [Member]    
Disclosure Of Financial Instruments [Line Items]    
Increase in financial derivative charges on interest rate before tax $ 78,000,000 $ 88,000,000
Interest rate risk [Member] | Floating interest rate [Member]    
Disclosure Of Financial Instruments [Line Items]    
Adjusted borrowings, interest rate 79.00% 82.00%
Interest rate risk [Member] | Fixed interest rate [Member]    
Disclosure Of Financial Instruments [Line Items]    
Adjusted borrowings, interest rate 21.00% 18.00%
Interest rate risk [Member] | LIBOR [member]    
Disclosure Of Financial Instruments [Line Items]    
Borrowings, interest rate basis 1.00%  
Borrowing cost recognised $ (8,000,000) $ 8,000,000
Interest rate risk [Member] | Interest rate swaps [member]    
Disclosure Of Financial Instruments [Line Items]    
Description of hedging strategy The Group hedges its interest rate risk by entering into interest rate derivatives to achieve its policies. The Group reviews the positions on a regular basis. During 2018, in conjunction with its liability management programme  
Notional amount $ 1,900,000,000 2,000,000,000
Inflows of cash from liability management programme 40,000,000 5,000,000
Accrued interest $ 11,000,000 $ 2,000,000