Benefit Plans - Net Pension and Postretirement Cost (Detail) - Pension Plans - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 40	$ 35
Interest cost	22	28
Expected return on plan assets	(49)	(47)
Amortization of prior service credit	(3)	(3)
Amortization of loss	25	20
Net pension and postretirement cost	$ 35	$ 32