Benefit Plans - Net Pension and Other Postretirement Cost (Detail) - Pension Plans - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 136	$ 110	$ 81
Interest cost	90	61	72
Expected return on plan assets	(154)	(112)	(109)
Amortization of prior service credit	(13)	(14)	(15)
Amortization of loss	78	92	77
Settlements	2	0	7
Net pension cost	137	138	113
Foreign Plans
Defined Benefit Plan Disclosure [Line Items]
Net pension cost	$ 34	$ 43	$ 35