Accumulated Other Comprehensive Income (Loss) - Reclassification out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance		$ 1,553.3	$ 1,694.2	$ 1,760.9	$ 1,862.3
Other comprehensive income before reclassifications		2.3	1.6	12.2	(16.9)
Balance		1,638.4	1,686.7	1,638.4	1,686.7
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance				(19.7)
Other comprehensive income before reclassifications				6.1
Balance		(13.6)		(13.6)
Derivative Financial Instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance				(15.0)
Other comprehensive income before reclassifications				6.1
Balance		(8.9)		(8.9)
AOCI [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	[1]	(24.8)	(39.1)	(34.7)	(20.6)
Other comprehensive income before reclassifications				12.2
Balance	[1]	$ (22.5)	$ (37.5)	$ (22.5)	$ (37.5)

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