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Debt - Schedule of Debt Obligations (Detail) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Total debt $ 680.0 $ 1,132.3
Less unamortized discount on Term Debt 1.8 0.9
Less unamortized debt issuance costs on Senior Notes and Term Debt 6.7 11.6
Less current portion of long-term debt 5.1  
Long-term debt, less current portion 666.4 1,119.8
Term Debt [Member]    
Debt Instrument [Line Items]    
Total debt $ 680.0 657.3
Senior Notes [Member]    
Debt Instrument [Line Items]    
Total debt   $ 475.0