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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 |

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