Goodwill and Intangible Assets - Components of Company's Goodwill and Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	$ 3,465.5	$ 3,456.4	$ 3,147.8
Intangible assets:
Customer relationships	1,040.4	1,038.4	782.1
Trademarks and tradenames	11.8	16.2	16.5
Noncompete agreements and other	68.1	46.4	55.2
Accumulated amortization	(478.7)	(441.8)	(416.3)
Intangible assets, net (definite-lived)	641.6	659.2	437.5
Trademarks and tradenames (indefinite-lived)	46.2	49.4	50.8
Total intangible assets, net	$ 687.8	$ 708.6	$ 488.3

Finite-Lived Trademarks and Tradenames, Gross

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Gross carrying amount before accumulated amortization as of the balance sheet date to an asset acquired in a business combination representing a favorable existing relationship with customers having a finite beneficial life.

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Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

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Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Gross carrying amount before accumulated amortization as of the balance sheet date of payments made to third parties in exchange for their agreement not to engage in specified competitive practices in specified geographic territories for a specified period of time.

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
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Carrying amount (original costs adjusted for previously recognized amortization and impairment) as of the balance sheet date for the rights acquired through registration of a trademark to gain or protect exclusive use of a business name, symbol or other device or style for a projected indefinite period of benefit.

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Sum of the carrying amounts of all intangible assets, excluding goodwill, as of the balance sheet date, net of accumulated amortization and impairment charges.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
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