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Dividends - Summary of Dividends (Detail) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
 [1]
Dec. 31, 2015
 [1]
Disclosure of Dividends [Line Items]      
Dividends recognised as distribution to owners $ 62.8 $ 39.2 $ 15.1
Dividends per share $ 0.08 $ 0.05 $ 0.02
Final Dividend [Member]      
Disclosure of Dividends [Line Items]      
Dividends recognised as distribution to owners $ 37.5 $ 10.6 $ 12.8
Interim Dividend [Member]      
Disclosure of Dividends [Line Items]      
Dividends recognised as distribution to owners $ 25.3 $ 28.6 $ 2.3
[1] As Restated - Refer note 40 for further details.