Benefit Plans - Net Pension and Other Postretirement Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 77	$ 71	$ 84
Interest cost	87	93	87
Expected return on plan assets	(123)	(126)	(116)
Amortization of prior service credit	(15)	(15)	(13)
Amortization of loss	68	49	75
Curtailment/settlement loss	0	3	6
Net pension and postretirement cost	93	74	123
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	3	6
Interest cost	7	9	10
Expected return on plan assets	0	0	0
Amortization of prior service credit	(5)	(4)	(1)
Amortization of loss	3	2	4
Curtailment/settlement loss	0	0	0
Net pension and postretirement cost	$ 9	$ 10	$ 19