Retirement Plans - Changes in the Fair Value of Pension Assets (Details) - Pension Plan - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Change in Plan Assets:
Fair value of plan assets at beginning of year	$ 449	$ 491
Fair value of plan assets at end of year	470	449
Significant Unobservable Inputs (Level 3)
Change in Plan Assets:
Fair value of plan assets at beginning of year	6
Fair value of plan assets at end of year	6	6
Real Estate [Member] | Significant Unobservable Inputs (Level 3)
Change in Plan Assets:
Fair value of plan assets at beginning of year	6	11
Redemptions	0	(5)
Fair value of plan assets at end of year	$ 6	$ 6