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Retirement Plans - Plan Assets by Asset Category (Details) - Pension plan - USD ($) $ in Millions | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | $ 486 | $ 470 | $ 449
Quoted Prices in Active Markets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 172 | 201 |
Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 235 | 202 |
Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 6 | 6 |
Net Asset Value (NAV) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 73 | 61 |
Cash |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 37 | 31 |
Cash | Quoted Prices in Active Markets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 37 | 31 |
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 | 15 | 15 |
Domestic | Quoted Prices in Active Markets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 1 | 2 |
Domestic | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 7 | 7 |
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 | 7 | 6 |
International - Developed |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 51 | 49 |
International - Developed | Quoted Prices in Active Markets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 29 | 36 |
International - Developed | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 11 | 10 |
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 | 11 | 3 |
International - Emerging |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 3 | 5 |
International - Emerging | Quoted Prices in Active Markets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 0 | 2 |
International - Emerging | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 3 | 3 |
International - Emerging | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 0 | 0 |
International - Emerging | Net Asset Value (NAV) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 0 | 0 |
Government |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 219 | 218 |
Government | Quoted Prices in Active Markets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 63 | 75 |
Government | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 128 | 117 |
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 | 28 | 26 |
Corporate/Other |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 64 | 66 |
Corporate/Other | Quoted Prices in Active Markets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 42 | 55 |
Corporate/Other | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 11 | 0 |
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 | 11 | 11 |
Hedge Fund |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | 75 | 65 |
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 | 75 | 65 |
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 | 22 | 21 |
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 | 6 | 6 |
Real Estate | Net Asset Value (NAV) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets, fair value | $ 16 | $ 15 |

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