Benefit Plans - Net Pension and Postretirement Cost (Detail) - Pension Plans - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 33	$ 43	$ 101	$ 107
Interest cost	26	29	80	70
Expected return on plan assets	(44)	(52)	(135)	(125)
Amortization of prior service credit	(3)	(3)	(10)	(10)
Amortization of loss	19	20	58	59
Settlements	0	4	0	6
Net pension and postretirement cost	$ 30	$ 39	$ 93	$ 107