Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Balance at beginning of the year	$ 27	$ 37	$ 36
Additions based on tax provisions related to the current year	2		2
Additions for tax positions of prior years	2		1
Reductions of tax provisions of prior years	(1)	(1)
Reductions related to settlements	(5)	(5)
Reductions from lapse of statute of limitations	(2)	(4)	(2)
Balance at end of the year	$ 23	$ 27	$ 37