Benefit Plans - Net Pension and Other Postretirement Cost (Detail) - Pension Plans - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 153	$ 134	$ 136
Interest cost	84	107	90
Expected return on plan assets	(188)	(180)	(154)
Amortization of prior service credit	(13)	(13)	(13)
Amortization of loss	97	78	78
Settlements	4	10	2
Net pension cost	137	135	137
Foreign Plans
Defined Benefit Plan Disclosure [Line Items]
Net pension cost	$ 41	$ 32	$ 34