Benefit Plans - Net Pension and Postretirement Cost (Detail) - Pension Plans - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 37	$ 33	$ 116	$ 101
Interest cost	21	26	64	80
Expected return on plan assets	(46)	(44)	(143)	(135)
Amortization of prior service credit	(3)	(3)	(10)	(10)
Amortization of loss	24	19	74	58
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement	2	0	2	0
Net pension and postretirement cost	$ 35	$ 30	$ 103	$ 93