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		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Earnings Per Share [Abstract]
Net income (loss)	$ (16.2)	$ 69.8	$ (17.2)	$ 99.6
Less: Preferred share dividends	2.7	2.7	5.3	5.3
Net income (loss) attributable to common shareholders	(18.9)	67.1	(22.5)	94.3
Net income (loss) attributable to common shareholders	$ (18.9)	$ 67.1	$ (22.5)	$ 94.3
Weighted average common shares outstanding - basic (in shares)	34,964,773	34,820,649	34,928,555	34,766,948
Effect of dilutive securities:
Equity compensation awards (in shares)	0	192,131	0	216,856
Weighted average common shares outstanding - diluted (in shares)	34,964,773	35,012,780	34,928,555	34,983,804
Net income (loss) per common share:
Basic (in dollars per share)	$ (0.54)	$ 1.93	$ (0.64)	$ 2.72
Diluted (in dollars per share)	$ (0.54)	$ 1.92	$ (0.64)	$ 2.70