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Long-Term Debt - Summary of Debt Refinancing Costs (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]      
Gross cost,Beginning balance $ 2.7 $ 1.5  
Capitalized in the year 0.0 1.2 $ 1.5
Gross cost,Ending Balance 2.7 2.7 1.5
Accumulated amortization,Beginning Balance (0.5) (0.1)  
Amortization in the year (0.6) (0.4)  
Accumulated amortization,Ending balance (1.1) (0.5) $ (0.1)
Deferred finance costs $ 1.6 $ 2.2