Benefit Plans - Net Pension and Other Postretirement Cost (Detail) - Pension Plans - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 134	$ 136	$ 110
Interest cost	107	90	61
Expected return on plan assets	(180)	(154)	(112)
Amortization of prior service credit	(13)	(13)	(14)
Amortization of loss	78	78	92
Settlements	10	2	0
Net pension cost	135	137	138
Foreign Plans
Defined Benefit Plan Disclosure [Line Items]
Net pension cost	$ 32	$ 34	$ 43