Stock-Based Compensation Plans - Schedule of SARs Activity (Details) - Stock appreciation rights - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
SARs outstanding at beginning of period (in shares)	1,824
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	228
Exercised (in shares)	(205)
Forfeited (in shares)	(49)
SARs outstanding at end of period (in shares)	1,798	1,824
SARs exercisable at end of period (in shares)	1,207
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
SARs outstanding at beginning of period, weighted average exercise price per share (USD per share)	$ 17.48
Granted, weighted average exercise price per share (USD per share)	23.08
Exercised, weighted average exercise price per share (USD per share)	13.64
Forfeited, weighted average exercise price per share (USD per share)	20.50
SARs outstanding at end of period, weighted average exercise price per share (USD per share)	18.54	$ 17.48
SARs exercisable at end of period, weighted average exercise price per share (USD per share)	$ 17.14
Outstanding, weighted average remaining term	6 years 8 months 12 days	7 years 1 month 6 days
Exercisable, weighted average remaining term	5 years 9 months 18 days
Outstanding, aggregate intrinsic value	$ 6	$ 11
Exercised, aggregate intrinsic value	2
Exercisable, aggregate intrinsic value	$ 5