Employee Benefit Plans - Components of Pension and Other Postretirement Benefit Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0	$ 0	$ 1	$ 1
Interest cost	21	19	61	56
Expected return on plan assets	(20)	(26)	(60)	(78)
Amortization of prior service credit	0	0	0	0
Net periodic benefit cost (income)	1	(7)	2	(21)
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	0	0	1	1
Expected return on plan assets	0	0	0	0
Amortization of prior service credit	(3)	(3)	(9)	(9)
Net periodic benefit cost (income)	$ (3)	$ (3)	$ (8)	$ (8)