Income Taxes - Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	$ 2,537	$ 2,726	$ 2,528
Additions for tax positions related to the current year	72	169	240
Additions for tax positions of prior years	96	31	33
Reductions for tax positions of prior years	(27)	(48)	(6)
Settlements with taxing authorities	(6)	(7)	(24)
Statute closings and audit resolutions	(446)	(334)	(45)
Ending balance	$ 2,226	$ 2,537	$ 2,726