Employee Benefit Plans - Schedule of Level 3 Investments (Details) - Pension benefits - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets at beginning of period	$ 2,081	$ 2,307
Actual return on assets held at end of year	(30)	148
Fair value of plan assets at end of period	1,792	2,081
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	16	23
Actual return on assets held at end of year		(7)
Purchases	3
Sales	(8)
Actual return on assets held at end of year	1
Actual return on assets sold during year	(8)
Fair value of plan assets at end of period	$ 4	$ 16