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Goodwill - Summary of Goodwill (Detail) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]    
Gross cost, beginning balance $ 611.3  
Movement in the period (39.4)  
Gross cost, ending balance 571.9  
Accumulated impairment losses (236.5) $ (236.5)
Net book amount, beginning balance 374.8  
Movement in the period (39.4)  
Net book amount, ending balance $ 335.4