Benefit Plans - Net Pension and Postretirement Cost (Detail) - Pension Plans - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 33	$ 34	$ 68	$ 64
Interest cost	26	22	54	41
Expected return on plan assets	(45)	(40)	(91)	(72)
Amortization of prior service credit	(3)	(3)	(7)	(7)
Amortization of loss	19	19	39	39
Settlements	0	2	0	2
Net pension and postretirement cost	$ 31	$ 35	$ 63	$ 67