Income Taxes - Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Income Tax Disclosure [Abstract]
Income tax expense attributable to income from operations	$ 84.5	$ 70.1	$ 37.8
Taxes allocated to stockholders’ equity, related to pension liabilities	0.2	(0.1)	0.5
Taxes allocated to stockholders’ equity, related to hedge accounting	11.1	(17.0)	0.0
Total income tax expense	$ 95.8	$ 53.0	$ 38.3