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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share [Abstract]
Net (loss) income	$ (29.6)	$ (25.1)	$ (54.8)	$ 94.9
Less: Preferred share dividends	2.0	0.0	2.0	0.0
Net (loss) income attributable to common shareholders	$ (31.6)	$ (25.1)	$ (56.8)	$ 94.9
Weighted average common shares outstanding - basic (in shares)	34,667,266	34,299,999	34,590,659	34,161,016
Effect of dilutive securities:
Equity compensation awards (in shares)	0	0	0	608,606
Weighted average common shares outstanding - diluted (in shares)	34,667,266	34,299,999	34,590,659	34,769,622
Net (loss) income per common share:
Basic (in dollars per share)	$ (0.91)	$ (0.73)	$ (1.64)	$ 2.78
Diluted (in dollars per share)	$ (0.91)	$ (0.73)	$ (1.64)	$ 2.73