Net Income (Loss) Per Share - Components and Calculation of Basic and Diluted Net Income (Loss) Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended								12 Months Ended
		Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2021		Sep. 30, 2020		Sep. 30, 2019
Numerator:
Net income (loss) from continuing operations		$ 104.5	$ 79.8	$ (10.5)	$ 47.4	$ 68.2	$ (4.1)	$ (121.4)	$ (24.0)	$ 221.2		$ (81.3)		$ (18.9)
Dividends on Preferred Stock										(24.0)	[1]	(24.0)		(24.0)
Net income (loss) from continuing operations attributable to common stockholders - Basic										197.2		(105.3)		(42.9)
Dividends on Preferred Stock										24.0	[1]	24.0		24.0
Net income (loss) from continuing operations attributable to common stockholders - Diluted										221.2		(105.3)		(42.9)
Net income (loss) from discontinued operations attributable to common stockholders - Basic and Diluted										(266.7)		0.4		8.3
Net income (loss) attributable to common stockholders - Basic										(69.5)		(104.9)		(34.6)
Net income (loss) attributable to common stockholders - Diluted										$ (45.5)		$ (104.9)		$ (34.6)
Denominator:
Weighted-average common shares outstanding - basic	[2]									69,700,000		68,800,000		68,400,000
Effect of common share equivalents										1,100,000
Effect of convertible Preferred Stock	[1]									9,700,000
Weighted-average common shares outstanding - diluted	[2]									80,500,000		68,800,000		68,400,000
Net income (loss) per share:
Basic - Continuing operations		$ 1.23	$ 0.93	$ (0.24)	$ 0.60	$ 0.79	$ (0.14)	$ (1.85)	$ (0.44)	$ 2.83	[2]	$ (1.53)	[2]	$ (0.63)	[2]
Basic - Discontinued operations	[2]									(3.83)		0.01		0.12
Basic net income (loss) per share		1.24	0.89	(0.18)	(3.27)	0.84	(0.18)	(1.87)	(0.43)	(1.00)	[2]	(1.52)	[2]	(0.51)	[2]
Diluted - Continuing operations		1.21	0.91	(0.24)	0.59	0.78	(0.14)	(1.85)	(0.44)	2.75	[2]	(1.53)	[2]	(0.63)	[2]
Diluted - Discontinued operations	[2]									(3.32)		0.01		0.12
Diluted net income (loss) per share		$ 1.22	$ 0.87	$ (0.18)	$ (3.24)	$ 0.83	$ (0.18)	$ (1.87)	$ (0.43)	$ (0.57)	[2]	$ (1.52)	[2]	$ (0.51)	[2]

[1] The hypothetical conversion of the Preferred Stock became dilutive for the year ended September 30, 2021, primarily stemming from the significant income from continuing operations and offsetting loss from discontinued operations in 2021, and their combined effect on the Company’s calculation of diluted net income (loss) per share.

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