Benefit Plans - Net Pension and Postretirement Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 27	$ 20	$ 51	$ 41
Interest cost	18	18	35	37
Expected return on plan assets	(33)	(27)	(63)	(55)
Amortization of prior service credit	(4)	(4)	(8)	(7)
Amortization of loss	28	19	52	39
Settlements	0	(1)	0	(1)
Net pension and postretirement cost	35	27	67	55
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1	1	1
Interest cost	1	1	2	3
Expected return on plan assets	0	0	0	0
Amortization of prior service credit	(1)	(1)	(2)	(2)
Amortization of loss	0	0	1	1
Settlements	0	0	0	0
Net pension and postretirement cost	$ 1	$ 1	$ 2	$ 3