Benefit Plans - Net Pension and Postretirement Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 27	$ 20	$ 79	$ 61
Interest cost	18	18	53	54
Expected return on plan assets	(34)	(27)	(97)	(82)
Amortization of prior service credit	(4)	(4)	(12)	(11)
Amortization of loss	28	19	80	58
Settlements	0	(3)	0	(4)
Net pension and postretirement cost	36	29	103	85
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1	2	2
Interest cost	1	1	3	4
Expected return on plan assets	0	0	0	0
Amortization of prior service credit	(1)	(1)	(4)	(4)
Amortization of loss	0	0	1	1
Settlements	0	0	0	0
Net pension and postretirement cost	$ 1	$ 1	$ 3	$ 4