Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Income tax expense	$ 137.3	$ 108.4	$ 228.1	$ 197.4
Effective tax rate (percent)	22.10%	23.40%	19.90%	23.50%
Effective Income Tax Rate Reconciliation, Tax Credit, Other, Amount	$ 6.6		$ 39.5
Deferred tax assets net of DTL	167.3		167.3		$ 181.8
Valuation allowance for deferred income taxes	$ 17.8		$ 17.8		$ 18.7