Retirement Plans - Plan Assets by Asset Category (Details) - Pension plan - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	$ 293	$ 263	$ 283
Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	88	78
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	157	134
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	4
Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	48	47
Cash
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	5	6
Cash | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	5	6
Cash | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Cash | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Cash | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Domestic
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	2	1
Domestic | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	2	1
Domestic | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Domestic | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Domestic | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
International - Developed
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	19	17
International - Developed | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	19	17
International - Developed | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
International - Developed | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
International - Developed | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
International - Emerging
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value		0
International - Emerging | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value		0
International - Emerging | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value		0
International - Emerging | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value		0
International - Emerging | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value		0
Government
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	150	128
Government | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	33	30
Government | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	90	75
Government | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Government | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	27	23
Corporate/Other
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	70	59
Corporate/Other | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	29	24
Corporate/Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	29	25
Corporate/Other | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Corporate/Other | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	12	10
Hedge Fund
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	38	34
Hedge Fund | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Hedge Fund | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	38	34
Hedge Fund | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Hedge Fund | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Real Estate
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	9	18
Real Estate | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Real Estate | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Real Estate | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	4
Real Estate | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	$ 9	$ 14