Goodwill and Intangible Assets - Summary of Intangible Assets (Detail) - USD ($) $ in Millions		9 Months Ended
		Jun. 30, 2021	Sep. 30, 2020	Jun. 30, 2020
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets		$ 1,086.3	$ 1,089.4	$ 1,091.6
Accumulated amortization		(656.3)	(581.2)	(553.2)
Total amortizable intangible assets, net		430.0	508.2	538.4
Indefinite-lived trademarks		9.8	9.8	9.8
Total intangibles, net		$ 439.8	518.0	548.2
Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		28 years 6 months
Minimum [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		5 years
Maximum [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		20 years
Noncompete Agreements [Member]
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets		$ 0.2	0.2	0.2
Noncompete Agreements [Member] | Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	[1]	10 months 24 days
Customer Relationships [Member]
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets		$ 1,086.1	1,085.5	1,087.7
Customer Relationships [Member] | Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	[1]	28 years 6 months
Trademarks [Member]
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets			$ 3.7	$ 3.7

[1] As of June 30, 2021.