Income Taxes - Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Income tax expense	$ 37.8	$ 37.1	$ 25.9
Taxes allocated to stockholders’ equity, related to pension liabilities	0.5	0.0	(0.2)
Total income tax expense	$ 38.3	$ 37.1	$ 25.7