Employee Benefit Plans - Pension and Other Postretirement Benefit Costs (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit income	$ (10)	$ (25)
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	1
Interest cost	19	21
Expected return on plan assets	(26)	(36)
Amortization of prior service credit	0	0
Actuarial gain	0	0
Net periodic benefit income	(7)	(14)
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0
Interest cost	0	0
Expected return on plan assets	0	0
Amortization of prior service credit	(3)	(3)
Actuarial gain	0	(8)
Net periodic benefit income	$ (3)	$ (11)