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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net (loss) income	$ 11.6	$ 11.5	$ (68.7)	$ 54.7
Denominator:
Weighted average shares for basic net earnings per common share (shares)	43.8	43.5	43.7	43.4
Effect of dilutive share-based awards (shares)	0.5	0.5	0.0	0.6
Adjusted weighted average shares for diluted net earnings per common share (shares)	44.3	44.0	43.7	44.0
Basic net (loss) earnings per common share (usd per share)	$ 0.26	$ 0.26	$ (1.57)	$ 1.26
Diluted net (loss) earnings per common share (usd per share)	$ 0.26	$ 0.25	$ (1.57)	$ 1.25