Income Taxes - Reconciliation of Expected Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Comparison of income tax expense (benefit) and tax computed at the statutory rate
Income taxes at federal statutory rate	$ 1,326.1	$ 1,602.2	$ 1,124.8
Increase (decrease) in tax resulting from:
State income taxes, net of federal benefit	208.1	210.0	166.9
Valuation allowance	(3.1)	(2.1)	(3.3)
Tax credits	(44.4)	(100.8)	(116.3)
Excess tax benefit from equity compensation	(25.6)	(20.1)	(38.4)
Effective Income Tax Rate Reconciliation, Tax Contingency, Amount	(1.5)	0.0	(6.0)
Other	59.9	44.9	37.4
Total income tax expense (benefit)	$ 1,519.5	$ 1,734.1	$ 1,165.1