Benefit Plans - Net Pension and Postretirement Cost (Detail) - Pension Plans - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 36	$ 39	$ 79	$ 79
Interest cost	17	21	37	43
Expected return on plan assets	(41)	(48)	(89)	(97)
Amortization of prior service credit	(3)	(3)	(7)	(7)
Amortization of loss	23	25	50	50
Net pension cost	$ 32	$ 34	$ 69	$ 68