Employee Benefit Plans - Components of Pension and Other Postretirement Benefit Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0	$ 1	$ 1	$ 1
Interest cost	15	20	31	40
Expected return on plan assets	(22)	(20)	(44)	(40)
Amortization of prior service credit	0	0	0	0
Net periodic benefit (income) cost	(7)	1	(12)	1
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	1	0	1	1
Expected return on plan assets	0	0	0	0
Amortization of prior service credit	(3)	(3)	(6)	(6)
Net periodic benefit (income) cost	$ (2)	$ (3)	$ (5)	$ (5)