Income Taxes - Summary of Components of Income Tax Provision (Benefit) (Detail) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Current:
Federal	[1]	$ (1.1)	$ 0.0	$ (4.4)
Foreign		1.4	0.6	0.5
State		0.6	1.6	3.5
Total current taxes		0.9	2.2	(0.4)
Deferred:
Federal		(21.3)	(1.5)	(35.2)
Foreign		0.2	0.0	(0.2)
State		(6.6)	(0.9)	5.3
Total deferred taxes		(27.7)	(2.4)	(30.1)
Provision for (benefit from) income taxes		$ (26.8)	$ (0.2)	$ (30.5)

[1] 2018 tax benefit due to changes in the treatment of acquired fixed assets stemming from the Tax Cut and Jobs Act of 2017