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Environmental Trust Funds - Schedule of Environmental Trust Funds (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Environmental Trust Funds [Abstract]      
Balance at beginning of the year $ 44.5 [1] $ 35.0  
Contributions from continuing operations 8.6 7.5  
Interest earned 0.5 1.0 [1] $ 0.4 [1]
Translation adjustment 1.9 1.0  
Balance at end of the year $ 55.5 $ 44.5 [1] $ 35.0
[1] As Restated - Refer note 40 for further details.