Benefit Plans - Net Pension and Postretirement Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 21	$ 19
Interest cost	19	22
Expected return on plan assets	(29)	(31)
Amortization of prior service credit	(4)	(4)
Amortization of loss	20	17
Net pension and postretirement cost	28	23
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1
Interest cost	1	2
Expected return on plan assets	0	0
Amortization of prior service credit	(1)	(1)
Amortization of loss	0	1
Net pension and postretirement cost	$ 1	$ 2