Intangible Assets, Goodwill and Acquisitions - Summary of Goodwill (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Mar. 31, 2022		Sep. 30, 2021
Goodwill [Line Items]
Beginning balance	$ 1,950.8		$ 1,859.3
Acquisitions	67.0	[1]	91.5
Ending balance	2,017.8		1,950.8
Maintenance Services
Goodwill [Line Items]
Beginning balance	1,758.0		1,680.4
Acquisitions	30.1	[1]	77.6
Ending balance	1,788.1		1,758.0
Development Services
Goodwill [Line Items]
Beginning balance	192.8		178.9
Acquisitions	36.9	[1]	13.9
Ending balance	$ 229.7		$ 192.8

[1] The acquisitions adjustment includes the immaterial impact of foreign currency adjustments during the period.