Net Income (Loss) Per Share - Components and Calculation of Basic and Diluted Net Income (Loss) Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net income (loss)		$ (80.9)	$ (10.6)	$ 98.6
Dividends on Preferred Stock		24.0	24.0	18.0
Net income (loss) attributable to common shareholders		(104.9)	(34.6)	80.6
Undistributed income allocated to participating securities				(7.7)
Net income (loss) attributable to common shareholders - basic and diluted		$ (104.9)	$ (34.6)	$ 72.9
Weighted-average common shares outstanding - basic	[1]	68,800,000	68,400,000	68,000,000.0
Effect of common share equivalents				1,200,000
Weighted-average common shares outstanding - diluted	[1]	68,800,000	68,400,000	69,200,000
Net income (loss) per share - basic	[1]	$ (1.52)	$ (0.51)	$ 1.07
Net income (loss) per share - diluted	[1]	$ (1.52)	$ (0.51)	$ 1.05

[1] See Note 5 for detailed calculations and further discussion.