Benefit Plans - Net Pension and Postretirement Cost (Detail) - Pension Plans - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 39	$ 33	$ 79	$ 68
Interest cost	21	26	43	54
Expected return on plan assets	(48)	(45)	(97)	(91)
Amortization of prior service credit	(3)	(3)	(7)	(7)
Amortization of loss	25	19	50	39
Net pension and postretirement cost	$ 34	$ 31	$ 68	$ 63