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, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Earnings Per Share [Abstract]
Net income (loss)	$ (48.8)	$ 22.4	$ (66.0)	$ 122.0
Less: Preferred share dividends	2.6	2.6	7.9	7.9
Net income (loss) attributable to common shareholders	(51.4)	19.8	(73.9)	114.1
Net income (loss) attributable to common shareholders	$ (51.4)	$ 19.8	$ (73.9)	$ 114.1
Weighted average common shares outstanding - basic (in shares)	35,014,182	34,852,274	34,955,787	34,794,078
Effect of dilutive securities:
Equity compensation awards (in shares)	0	181,976	0	265,888
Weighted average common shares outstanding - diluted (in shares)	35,014,182	35,034,250	34,955,787	35,059,966
Net income (loss) per common share:
Basic (in dollars per share)	$ (1.47)	$ 0.57	$ (2.11)	$ 3.28
Diluted (in dollars per share)	$ (1.47)	$ 0.56	$ (2.11)	$ 3.26