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Pension Benefit Obligations - Asset Categories (Details) - USD ($) $ in Millions | Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2019
--- | --- | --- | ---
Post-retirement plan |  |  |
Fair values of plan assets | $ 1,390.7 | $ 1,295.8 |
Cash and cash equivalents |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 49.8 | 50.6 |
Equity and debt securities |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 488.5 | 442.3 |
Diversified and equity funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 81.2 | 31.5 |
Fixed income funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 29.3 | 36.2 |
Common collective funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 734.8 | 707.5 |
Derivative instruments |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 7.1 | 27.7 |
Level 1 |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 599.1 | 498.6 |
Level 1 | Cash and cash equivalents |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 25.1 | 20.2 |
Level 1 | Equity and debt securities |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 488.5 | 442.3 |
Level 1 | Diversified and equity funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 60.5 | 13.0 |
Level 1 | Fixed income funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 25.0 | 23.1 |
Level 2 |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 52.8 | 86.3 |
Level 2 | Cash and cash equivalents |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 24.7 | 30.4 |
Level 2 | Diversified and equity funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 16.7 | 15.1 |
Level 2 | Fixed income funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 4.3 | 13.1 |
Level 2 | Derivative instruments |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 7.1 | 27.7 |
Level 3 |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 4.0 | 3.4 | $ 26.8
Level 3 | Diversified and equity funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 4.0 | 3.4 |
Investments measured at NAV |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 734.8 | 707.5 |
Investments measured at NAV | Common collective funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | $ 734.8 | $ 707.5 |

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