Income Taxes - Schedule of Components of Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Current:
Federal	$ (28.1)	$ 12.5	$ 10.1
State	3.8	5.0	5.0
Deferred:
Federal	28.4	(19.6)	0.8
State	0.5	(7.5)	(3.1)
Total income tax (benefit) expense	$ 4.6	$ (9.6)	$ 12.8