Goodwill and Intangible Assets - Summary of Intangible Assets (Detail) - USD ($) $ in Millions		6 Months Ended
		Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets		$ 1,086.2	$ 1,089.4	$ 1,095.0
Accumulated amortization		(631.0)	(581.2)	(525.7)
Total amortizable intangible assets, net		455.2	508.2	569.3
Indefinite-lived trademarks		9.8	9.8	9.8
Total intangibles, net		$ 465.0	518.0	579.1
Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		31 years 9 months 18 days
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]	1 year 2 months 12 days
Customer Relationships [Member]
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets		$ 1,086.0	1,085.5	1,090.7
Customer Relationships [Member] | Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	[1]	31 years 9 months 18 days
Trademarks [Member]
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets			$ 3.7	$ 4.1

[1] As of March 31, 2021.