Benefit Plans - Net Pension and Postretirement Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 20	$ 20	$ 61	$ 58
Interest cost	18	22	54	66
Expected return on plan assets	(27)	(32)	(82)	(93)
Amortization of prior service credit	(4)	(4)	(11)	(12)
Amortization of loss	19	18	58	52
Settlements	3	0	4	0
Net pension and postretirement cost	29	24	85	70
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1	2	2
Interest cost	1	2	4	6
Expected return on plan assets	0	0	0	0
Amortization of prior service credit	(1)	(1)	(4)	(4)
Amortization of loss	0	1	1	2
Settlements	0	0	0	0
Net pension and postretirement cost	$ 1	$ 2	$ 4	$ 7