Employee Retirement Plans - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Expected amortization of net actuarial losses	$ 0
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected amortization of net actuarial losses	15
ABO for the pension plans	687	$ 659
Contribution made to Pension Plan	13	12
Projected benefit obligations of unfunded and non qualified supplemental executive retirement plans	782	750	$ 669
Net actuarial loss	24	26
Fair value of pension and other postretirement benefit contributions	0	0
Costs associated with the plans	11	9	16
Supplemental Employee Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations of unfunded and non qualified supplemental executive retirement plans	59	53
Net actuarial loss	12	9
Fair value of pension and other postretirement benefit contributions	35	34
Costs associated with the plans	0	0
U.S. | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Contribution made to Pension Plan	$ 13	$ 12	$ 15