Goodwill and Intangible Assets - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Goodwill And Intangible Assets [Line Items]
Amortization			$ 103.3	$ 261.9	$ 142.9
Minimum [Member]
Goodwill And Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life			5 years
Maximum [Member]
Goodwill And Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life			20 years
Weighted Average [Member]
Goodwill And Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life			15 years 4 months 24 days
Customer Relationships [Member] | Weighted Average [Member]
Goodwill And Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	[1]		15 years 4 months 24 days
Allied Acquisition [Member] | Tradenames [Member]
Goodwill And Intangible Assets [Line Items]
Amortization		$ 142.6

[1] As of September 30, 2021.