Benefit Plans - Net Pension and Postretirement Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 20	$ 18	$ 58	$ 53
Interest cost	22	23	66	69
Expected return on plan assets	(32)	(32)	(93)	(94)
Amortization of prior service credit	(4)	(4)	(12)	(11)
Amortization of loss	18	12	52	36
Net pension and postretirement cost	24	18	70	53
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1	2	3
Interest cost	2	2	6	7
Amortization of prior service credit	(1)	(1)	(4)	(3)
Amortization of loss	1		2	2
Net pension and postretirement cost	$ 2	$ 2	$ 7	$ 8