Goodwill and Intangible Assets - Summary of Changes in goodwill (Detail) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2020	Sep. 30, 2019
Goodwill And Intangible Assets Disclosure [Abstract]
Beginning balance		$ 2,490.6	$ 2,491.8
Acquisitions	[1]		(0.5)
Translation and other adjustments		(0.2)	(0.7)
Ending balance		$ 2,490.4	$ 2,490.6

[1] Reflects purchase accounting adjustments related to fiscal year 2018 acquisition of Atlas Supply, Inc. (see Note 3 for further discussion).