Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Reconciliation of Unrecognized Tax Benefits
Unrecognized tax benefits — beginning of year	$ 11	$ 12	$ 12
Additions for tax positions of the current year	2	1	2
Additions for tax positions taken in prior years	0	0	1
Settlements with tax authorities/statute lapses	(9)	(2)	(3)
Unrecognized tax benefits — end of year	$ 4	$ 11	$ 12