Income Taxes - Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Current tax expense (benefit):
Federal	$ 1,293.0	$ 1,448.9	$ 978.1
State	272.4	256.1	197.0
Total current tax expense (benefit)	1,565.4	1,705.0	1,175.1
Deferred tax expense:
Federal	(39.0)	21.2	(12.7)
State	(6.9)	7.9	2.7
Total deferred tax expense (benefit)	(45.9)	29.1	(10.0)
Total income tax expense (benefit)	$ 1,519.5	$ 1,734.1	$ 1,165.1