Income Taxes - Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income tax expense attributable to income from operations	$ 2.5	$ 12.4	$ 16.8	$ 5.4	$ 6.9	$ 5.3	$ 7.5	$ 6.2	$ 37.1	$ 25.9	$ 46.0
Taxes allocated to stockholders’ equity, related to pension liabilities									0.0	(0.2)	0.1
Total income tax expense									$ 37.1	$ 25.7	$ 46.1