Intangible Assets, Goodwill and Acquisitions - Summary of Goodwill (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Dec. 31, 2020	Sep. 30, 2020	Sep. 30, 2020
Goodwill [Line Items]
Beginning balance	$ 1,859.3		$ 1,810.4
Acquisitions	41.0		64.4
Impairment Loss		$ (15.5)	(15.5)
Ending balance	1,900.3	1,859.3	1,859.3
Maintenance Services
Goodwill [Line Items]
Beginning balance	1,680.4		1,616.0
Acquisitions	39.8		64.4
Ending balance	1,720.2	1,680.4	1,680.4
Development Services
Goodwill [Line Items]
Beginning balance	178.9		194.4
Acquisitions	1.2
Impairment Loss			(15.5)
Ending balance	$ 180.1	$ 178.9	$ 178.9

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.

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