Employee Benefit Plans - Schedule of Level 3 Investments (Details) - Pension benefits - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets at beginning of period	$ 1,792	$ 2,081
Fair value of plan assets at end of period	1,943	1,792
Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets at beginning of period	4	16
Purchases	1	3
Sales		(8)
Actual return on assets held at end of year	2	1
Actual return on assets sold during year		(8)
Fair value of plan assets at end of period	$ 7	$ 4