Employee Benefit Plans - Plan Assets (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	$ 351	$ 347
Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	6	7
Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	345	340
Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	0	0
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	6	7
Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	6	7
Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	0	0
Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	0	0
U.S. Government Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	18	20
U.S. Government Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	0	0
U.S. Government Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	18	20
U.S. Government Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	0	0
Non-U.S. Government Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	74	80
Non-U.S. Government Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	0	0
Non-U.S. Government Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	74	80
Non-U.S. Government Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	0	0
Corporate Debt Instruments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	149	144
Corporate Debt Instruments [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	0	0
Corporate Debt Instruments [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	149	144
Corporate Debt Instruments [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	0	0
Listed Real Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	16	13
Listed Real Assets [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	0	0
Listed Real Assets [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	16	13
Listed Real Assets [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	0	0
Asset-backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	22	21
Asset-backed Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	0	0
Asset-backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	22	21
Asset-backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	0	0
Corporate Stocks [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	41	36
Corporate Stocks [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	0	0
Corporate Stocks [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	41	36
Corporate Stocks [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	0	0
Insurance Contracts [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	25	26
Insurance Contracts [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	0	0
Insurance Contracts [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	25	26
Insurance Contracts [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets for plan benefits	$ 0	$ 0