Earnings Per Common Share - Compilation of Basic and Diluted Earnings (Loss) Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income	$ 33.4	$ (9.5)	$ 37.3	$ (4.1)
Denominator:
Weighted average shares for basic net earnings per common share (shares)	43.0	42.2	42.8	42.0
Effect of dilutive share-based awards (shares)	0.6	0.0	0.5	0.0
Adjusted weighted average shares for diluted net earnings per common share (shares)	43.6	42.2	43.3	42.0
Basic net (loss) earnings per common share (usd per share)	$ 0.78	$ (0.23)	$ 0.87	$ (0.10)
Diluted net (loss) earnings per common share (usd per share)	$ 0.76	$ (0.23)	$ 0.86	$ (0.10)