Benefit Plans - Net Pension and Other Postretirement Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 81	$ 77	$ 71
Interest cost	72	87	93
Expected return on plan assets	(109)	(123)	(126)
Amortization of prior service credit	(15)	(15)	(15)
Amortization of loss	77	68	49
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements	(7)	0	(3)
Net pension and postretirement cost	113	93	74
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	3	3
Interest cost	5	7	9
Expected return on plan assets	0	0	0
Amortization of prior service credit	(5)	(5)	(4)
Amortization of loss	2	3	2
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements	0	0	0
Net pension and postretirement cost	$ 5	$ 9	$ 10