Employee Benefit Plans - Components of Pension and Other Postretirement Benefit Income (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1	$ 0
Interest cost	16	20
Expected return on plan assets	(22)	(20)
Amortization of prior service credit	0	0
Net periodic benefit income	(5)	0
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0
Interest cost	0	1
Expected return on plan assets	0	0
Amortization of prior service credit	(3)	(3)
Net periodic benefit income	$ (3)	$ (2)