Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Unrecognized Tax Benefits [Roll Forward]
Beginning balance	$ 499	$ 499	$ 483
Additions for tax positions related to the current year	2	62	29
Additions for tax positions of prior years	50	2	11
Reductions for tax positions of prior years	(5)	(52)	(9)
Settlements with taxing authorities	(11)	(3)	(12)
Statute closings	(8)	(9)	(3)
Ending balance	$ 527	$ 499	$ 499