Employee Benefit Plans - Components of Pension and Other Postretirement Benefit Income (Details) - 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]
Net periodic benefit income	$ (10)	$ (16)	$ (29)	$ (58)
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	1	1	2
Interest cost	19	22	56	65
Expected return on plan assets	(26)	(36)	(78)	(109)
Amortization of prior service credit	0	0	0	0
Actuarial gain			0	0
Net periodic benefit income	(7)	(13)	(21)	(42)
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)
Actuarial gain			0	(8)
Net periodic benefit income	$ (3)	$ (3)	$ (8)	$ (16)