Income Taxes - Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Current tax expense (benefit):
Federal	$ 978.1	$ 484.0	$ 407.3
State	197.0	104.4	79.3
Total current tax expense (benefit)	1,175.1	588.4	486.6
Deferred tax expense:
Federal	(12.7)	20.0	13.9
State	2.7	(5.9)	6.2
Total deferred tax expense (benefit)	(10.0)	14.1	20.1
Total income tax expense (benefit)	$ 1,165.1	$ 602.5	$ 506.7
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent		0.40%