Earnings Per Common Share - Compilation of Basic and Diluted Earnings (Loss) Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Numerator:
Net (loss) income	$ (114.6)	$ 49.4
Denominator:
Weighted average shares for basic net earnings per common share (shares)	43,700,000	43.3
Effect of dilutive share-based awards (shares)	0	0.6
Adjusted weighted average shares for diluted net earnings per common share (shares)	43,700,000	43.9
Basic net (loss) earnings per common share (usd per share)	$ (2.62)	$ 1.14
Diluted net (loss) earnings per common share (usd per share)	$ (2.62)	$ 1.12