Stock-Based Compensation Plans - Schedule of Nonvested Stock Award Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Nonvested Stock Units
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares
Unvested shares, beginning balance (in shares)	1,668
Granted (in shares)	520
Expected performance adjustments	(126)
Vested (in shares)	(288)
Forfeitures (in shares)	(58)
Unvested shares, ending balance (in shares)	1,968	1,668
Unvested shares ending balance, weighted average grant date fair value (usd per share)	$ 21.50	$ 20.89
Granted, weighted average grant date fair value (in usd per share)	22.33	$ 22.17	$ 20.72
Share-based Compensation Arrangement by Share Based Award, Equity Instruments Other Than Options Performance Shares weighted Average	22.96
Vested, weighted average grant date fair value (in usd per share)	22.69
Forfeitures, weighted average grant date fair value (in usd per share)	$ 21.85
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value
Grant date fair values of shares vested	$ 7	$ 5	$ 12
Unvested shares beginning balance, weighted average grant date fair value (usd per share)	$ 20.89
Nonvested Stock Units | Minimum
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value
Award vesting period	1 year
Nonvested Stock Units | Maximum
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value
Award vesting period	3 years
Nonvested Stock Units - Performance Based
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value
Award vesting period	3 years
Minimum risk free interest rate	0.13%	1.55%	2.66%
Maximum risk free interest rate	0.23%	1.59%	2.82%