Benefit Plans - Net Pension and Postretirement Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 24	$ 21
Interest cost	16	19
Expected return on plan assets	(30)	(29)
Amortization of prior service credit	(4)	(4)
Amortization of loss	25	20
Net pension and postretirement cost	32	28
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1
Interest cost	1	1
Expected return on plan assets	0	0
Amortization of prior service credit	(1)	(1)
Amortization of loss	0	0
Net pension and postretirement cost	$ 1	$ 1