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
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Net (loss) income	$ (18.8)	$ 91.2
Weighted average common shares outstanding - basic (in shares)	34,469,516	33,984,329
Effect of dilutive securities:
Equity compensation awards (in shares)	0	753,610
Weighted average common shares outstanding - diluted (in shares)	34,469,516	34,737,939
Net (loss) income per common share:
Basic (in dollars per share)	$ (0.55)	$ 2.68
Diluted (in dollars per share)	$ (0.55)	$ 2.63