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Pension Benefit Obligations - Asset Categories (Details) - USD ($) $ in Millions | Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021
--- | --- | --- | ---
Post-retirement plan |  |  |
Fair values of plan assets | $ 772.1 | $ 784.9 |
Cash and Cash Equivalents |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 29.5 | 104.8 |
Debt securities |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 338.3 | 339.1 |
Diversified and equity funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 43.5 | 22.2 |
Fixed income funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 26.1 | 7.9 |
Common collective funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 372.0 | 451.6 |
Derivative instruments |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | (37.3) | (140.7) |
Quoted Prices in Active Markets (Level 1) |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 412.0 | 451.1 |
Quoted Prices in Active Markets (Level 1) | Cash and Cash Equivalents |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 20.2 | 99.0 |
Quoted Prices in Active Markets (Level 1) | Debt securities |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 338.3 | 339.1 |
Quoted Prices in Active Markets (Level 1) | Diversified and equity funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 30.3 | 7.3 |
Quoted Prices in Active Markets (Level 1) | Fixed income funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 21.7 | 5.7 |
Quoted Prices in Active Markets (Level 1) | Derivative instruments |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 1.5 |  |
Significant Other Observable Inputs (Level 2) |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | (11.9) | (117.8) |
Significant Other Observable Inputs (Level 2) | Cash and Cash Equivalents |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 9.3 | 5.8 |
Significant Other Observable Inputs (Level 2) | Diversified and equity funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 13.2 | 14.9 |
Significant Other Observable Inputs (Level 2) | Fixed income funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 4.4 | 2.2 |
Significant Other Observable Inputs (Level 2) | Derivative instruments |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | (38.8) | (140.7) |
Significant Unobservable Inputs (Level 3) |  |  |
Post-retirement plan |  |  |
Fair values of plan assets |  |  | $ 4.0
Investments measured at NAV |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 372.0 | 451.6 |
Investments measured at NAV | Common collective funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | $ 372.0 | $ 451.6 |

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