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		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Net (loss) income	$ (6.4)	$ 28.8	$ (25.2)	$ 120.0
Weighted average common shares outstanding - basic (in shares)	34,634,353	34,195,250	34,551,935	34,090,372
Effect of dilutive securities:
Equity compensation awards (in shares)	0	584,741	0	669,176
Weighted average common shares outstanding - diluted (in shares)	34,634,353	34,779,991	34,551,935	34,759,548
Net (loss) income per common share:
Basic (in dollars per share)	$ (0.18)	$ 0.84	$ (0.73)	$ 3.52
Diluted (in dollars per share)	$ (0.18)	$ 0.83	$ (0.73)	$ 3.45