Net (Loss) Income Per Common Share - Net Income (Loss) Per Common Share on Basic and Diluted Basis (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net (loss) income	$ (25.1)	$ 40.6	$ 94.9	$ 107.2
Weighted average common shares outstanding - basic (in shares)	34,299,999	33,956,695	34,161,016	33,921,632
Effect of dilutive securities:
Equity compensation awards (in shares)	0	699,633	608,606	767,211
Weighted average common shares outstanding - diluted (in shares)	34,299,999	34,656,328	34,769,622	34,688,843
Net (loss) income per common share:
Basic (in dollars per share)	$ (0.73)	$ 1.20	$ 2.78	$ 3.16
Diluted (in dollars per share)	$ (0.73)	$ 1.17	$ 2.73	$ 3.09