Intangible Assets, Goodwill and Acquisitions - Schedule of Intangible Assets (Details) - USD ($) $ in Millions	6 Months Ended
	Mar. 31, 2021	Sep. 30, 2020
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 692.0	$ 672.8
Accumulated Amortization	(477.9)	(451.5)
Customer Relationships
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	685.5	666.3
Accumulated Amortization	$ (474.4)	(448.3)
Customer Relationships | Minimum
Finite Lived Intangible Assets [Line Items]
Useful Life	6 years
Customer Relationships | Maximum
Finite Lived Intangible Assets [Line Items]
Useful Life	21 years
Trademarks
Finite Lived Intangible Assets [Line Items]
Useful Life	12 years
Gross Carrying Amount	$ 3.8	3.8
Accumulated Amortization	(2.1)	(2.0)
Noncompete Agreements
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	2.7	2.7
Accumulated Amortization	$ (1.4)	$ (1.2)
Noncompete Agreements | Minimum
Finite Lived Intangible Assets [Line Items]
Useful Life	5 years
Noncompete Agreements | Maximum
Finite Lived Intangible Assets [Line Items]
Useful Life	7 years