Benefit Plans - Net Pension and Postretirement Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 20	$ 19	$ 41	$ 38
Interest cost	18	21	37	43
Expected return on plan assets	(27)	(30)	(55)	(61)
Amortization of prior service credit	(4)	(4)	(7)	(8)
Amortization of loss	19	17	39	34
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements	1	0	1	0
Net pension and postretirement cost	27	23	55	46
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1	1	2
Interest cost	1	2	3	4
Expected return on plan assets	0	0	0	0
Amortization of prior service credit	(1)	(1)	(2)	(2)
Amortization of loss	0	1	1	1
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements	0	0	0	0
Net pension and postretirement cost	$ 1	$ 2	$ 3	$ 4