Intangible Assets, Goodwill and Acquisitions - Summary of Goodwill (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Mar. 31, 2020	Sep. 30, 2019
Goodwill [Line Items]
Beginning balance	$ 1,810.4	$ 1,766.8
Acquisitions	68.5	43.6
Ending balance	1,878.9	1,810.4
Maintenance Services
Goodwill [Line Items]
Beginning balance	1,616.0	1,572.4
Acquisitions	68.5	43.6
Ending balance	1,684.5	1,616.0
Development Services
Goodwill [Line Items]
Beginning balance	194.4	194.4
Ending balance	$ 194.4	$ 194.4

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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
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