Benefit Plans - Net Pension and Postretirement Cost (Detail) - Pension Plans - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 43	$ 40
Interest cost	20	22
Expected return on plan assets	(48)	(49)
Amortization of prior service credit	(4)	(3)
Amortization of loss	27	25
Net pension cost	$ 38	$ 35