Accumulated Other Comprehensive Income (Loss) - Reclassification out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance		$ 1,862.3	$ 1,884.2	$ 1,781.8
Other comprehensive income (loss) before reclassifications		(14.1)	(3.3)	(2.7)
Balance		1,760.9	1,862.3	1,884.2
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance		(19.0)	(17.3)	(14.6)
Other comprehensive income (loss) before reclassifications		(0.7)	(1.7)	(2.7)
Balance		(19.7)	(19.0)	(17.3)
Derivative Financial Instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance		(1.6)
Other comprehensive income (loss) before reclassifications		(13.4)	(1.6)
Balance		(15.0)	(1.6)
AOCI [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	[1]	(20.6)	(17.3)	(14.6)
Other comprehensive income (loss) before reclassifications		(14.1)	(3.3)	(2.7)
Balance	[1]	$ (34.7)	$ (20.6)	$ (17.3)

[1] Accumulated Other Comprehensive Income (Loss) ("AOCI")