Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Income Tax Disclosure [Abstract]
Balance at beginning of the year	$ 159	$ 21	$ 23
Additions based on tax positions related to the current year	1	138	1
Additions for tax positions of prior years		2
Reductions of tax positions of prior years	(137)	(1)	(2)
Reductions related to settlements	(2)
Reductions from lapse of statute of limitations	(1)	(1)	(1)
Balance at end of the year	$ 20	$ 159	$ 21