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Pension Benefit Obligations - Asset Categories (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
--- | --- | --- | ---
Post-retirement plan |  |  |
Fair values of plan assets | $ 1,295.8 | $ 1,198.1 |
Cash and cash equivalents |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 50.6 | 35.7 |
Equity and debt securities |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 442.3 | 115.5 |
Diversified and equity funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 31.5 | 155.7 |
Fixed Income Funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 36.2 | 36.6 |
Common collective funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 707.5 | 668.7 |
Assets held by Insurance Company |  |  |
Post-retirement plan |  |  |
Fair values of plan assets |  | 26.8 |
Derivative instruments |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 27.7 | 159.1 |
Level 1 |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 498.6 | 299.8 |
Level 1 | Cash and cash equivalents |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 20.2 | 21.1 |
Level 1 | Equity and debt securities |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 442.3 | 115.5 |
Level 1 | Diversified and equity funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 13.0 | 141.9 |
Level 1 | Fixed Income Funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 23.1 | 21.3 |
Level 2 |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 86.3 | 202.8 |
Level 2 | Cash and cash equivalents |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 30.4 | 14.6 |
Level 2 | Diversified and equity funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 15.1 | 13.8 |
Level 2 | Fixed Income Funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 13.1 | 15.3 |
Level 2 | Derivative instruments |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 27.7 | 159.1 |
Level 3 |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 3.4 | 26.8 | $ 45.0
Level 3 | Diversified and equity funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 3.4 |  |
Level 3 | Assets held by Insurance Company |  |  |
Post-retirement plan |  |  |
Fair values of plan assets |  | 26.8 |
Investments measured at NAV |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 707.5 | 668.7 |
Investments measured at NAV | Common collective funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | $ 707.5 | $ 668.7 |

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