Benefit Plans - Net Pension and Postretirement Cost (Detail) - Pension Plans - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 43	$ 27	$ 107	$ 79
Interest cost	29	18	70	53
Expected return on plan assets	(52)	(34)	(125)	(97)
Amortization of prior service credit	(3)	(4)	(10)	(12)
Amortization of loss	20	28	59	80
Settlements	4	0	6	0
Net pension and postretirement cost	$ 39	$ 36	$ 107	$ 103