Net Income (Loss) 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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net income (loss)	$ 22.4	$ (23.1)	$ 122.0	$ (53.2)
Less: Preferred share dividends	2.6	2.0	7.9	2.0
Net income (loss) attributable to common shareholders	19.8	(25.1)	114.1	(55.2)
Net income (loss) attributable to common shareholders	$ 19.8	$ (25.1)	$ 114.1	$ (55.2)
Weighted average common shares outstanding - basic (in shares)	34,854,724	34,667,266	34,796,528	34,590,659
Effect of dilutive securities:
Equity compensation awards (in shares)	181,976	0	265,888	0
Weighted average common shares outstanding - diluted (in shares)	35,036,700	34,667,266	35,062,416	34,590,659
Net income (loss) per common share:
Basic (in dollars per share)	$ 0.57	$ (0.72)	$ 3.28	$ (1.60)
Diluted (in dollars per share)	$ 0.56	$ (0.72)	$ 3.26	$ (1.60)