Income Taxes - Reconciliation 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
Unrecognized tax benefits — beginning of year	$ 5	$ 3	$ 4
Additions for tax positions of the current year	3	2	0
Increases for tax positions taken in prior years	0	0	(2)
Increases for tax positions taken in prior years	19	1	1
Settlements with tax authorities/statute lapses	(1)	(1)	0
Unrecognized tax benefits — end of year	$ 26	$ 5	$ 3