Employee Benefit Plans - Schedule of Weighted-Average Asset Allocations (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Weighted average asset allocation (as a percent)	100.00%	100.00%
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	$ 72	$ 22	$ (69)
Nonvested Stock Units
Defined Benefit Plan Disclosure [Line Items]
Grant date fair values of shares vested	$ 7	$ 5	$ 12
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Weighted average asset allocation (as a percent)	11.00%	18.00%
Debt securities
Defined Benefit Plan Disclosure [Line Items]
Weighted average asset allocation (as a percent)	85.00%	80.00%
Other
Defined Benefit Plan Disclosure [Line Items]
Weighted average asset allocation (as a percent)	4.00%	2.00%
Minimum | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations	15.00%
Minimum | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations	65.00%
Minimum | Other
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations	0.00%
Maximum | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations	25.00%
Maximum | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations	85.00%
Maximum | Other
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations	20.00%