Benefit Plans - Net Pension and Postretirement Cost (Detail) - Pension Plans - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 30	$ 24
Interest cost	18	16
Expected return on plan assets	(33)	(30)
Amortization of prior service credit	(3)	(4)
Amortization of loss	20	25
Net pension and postretirement cost	$ 32	$ 32