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Dividends - Summary of Dividends Paid And Proposed (Detail) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Disclosure of Dividends Paid and Proposed [abstract]    
Final (declared for previous year) $ 78.1 $ 71.0
Interim 39.9 36.3
Special 262.1  
Dividends paid 380.1 107.3
Final Proposed (not recognised as a liability at 31 December) $ 85.9 $ 78.1
Final (declared for previous year) $ 139 $ 130
Interim 72 69
Special 510  
Dividends paid $ 721 199
Final Proposed (not recognised as a liability at 31 December)   $ 141