Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Income tax expense	$ 246.0	$ 137.3	$ 485.1	$ 228.1
Effective tax rate (percent)	20.80%	22.10%	21.90%	19.90%
Effective Income Tax Rate Reconciliation, Tax Credit, Other, Percent	0.60%		0.30%	2.60%
Deferred tax assets net of DTL	$ 150.2		$ 150.2		$ 152.4
Valuation allowance for deferred income taxes	$ 7.4		$ 7.4		$ 7.5