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Provisions - Summary of Silicosis Settlement Costs (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
 [1]
Provisions [abstract]      
Provision raised $ 30.2 $ 0.0 [1] $ 0.0
Unwinding of provision recognised as finance expense 0.9    
Translation 0.8 0.0  
Balance at end of the year $ 31.9 $ 0.0  
[1] As Restated - Refer note 40 for further details.