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Financial risk management and derivatives - Summary of Interest and Foreign Exchange Risk Sensitivities (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Impact on profit before tax [member] | Currency risk [member] | Sterling US dollar exchange rate [member]      
Disclosure of detailed information about financial instruments [Line Items]      
Impact of 5 cent fall in exchange rate $ 4.1 $ 4.0 $ 5.2
Impact on profit before tax [member] | Currency risk [member] | Euro US dollar exchange rate [member]      
Disclosure of detailed information about financial instruments [Line Items]      
Impact of 5 cent fall in exchange rate (2.4) (2.1) (2.2)
Impact on profit before tax [member] | Interest rate risk [member] | US dollar interest rates [member]      
Disclosure of detailed information about financial instruments [Line Items]      
Impact of 1% increase interest rates (0.9) (2.9) (1.8)
Impact on profit before tax [member] | Interest rate risk [member] | Sterling interest rates [member]      
Disclosure of detailed information about financial instruments [Line Items]      
Impact of 1% increase interest rates 5.5 0.3 1.3
Impact on net liabilities [member] | Currency risk [member] | Sterling US dollar exchange rate [member]      
Disclosure of detailed information about financial instruments [Line Items]      
Impact of 5 cent fall in exchange rate 25.8 44.1 47.2
Impact on net liabilities [member] | Currency risk [member] | Euro US dollar exchange rate [member]      
Disclosure of detailed information about financial instruments [Line Items]      
Impact of 5 cent fall in exchange rate 23.1 $ (4.1) $ (5.5)
Impact on net liabilities [member] | Currency risk [member] | Sterling: Euro exchange rate [member]      
Disclosure of detailed information about financial instruments [Line Items]      
Impact of 5 cent fall in exchange rate $ 31.9