Income Taxes - Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Current federal tax expense (benefit)	$ 10.7	$ 33.6	$ 29.4
Current state and local tax expense (benefit)	2.0	5.0	2.6
Current foreign tax expense (benefit)	(0.2)	0.3	0.3
Current income tax expense (benefit), total	12.5	38.9	32.3
Deferred federal income tax expense (benefit)	(2.0)	0.2	(9.1)
Deferred state and local income tax expense (benefit)	(1.3)	0.3	0.5
Deferred foreign income tax expense (benefit)	0.0	0.0	0.0
Deferred income taxes, total	(3.3)	0.5	(8.6)
Income tax expense, total	$ 9.2	$ 39.4	$ 23.7