Benefit Plans - Changes in Benefit Obligation for Postemployment Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Change in benefit obligation:
Beginning obligation	$ 163	$ 184
Service cost	23	18	$ 20
Interest cost	4	6	7
Benefits paid	(21)	(25)
Actuarial (gain) loss	0	(22)
Benefit obligation at September 30	$ 168	$ 163	$ 184