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Pension Benefit Obligations - Asset Categories (Details) - USD ($) $ in Millions | Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
--- | --- | --- | ---
Post-retirement plan |  |  |
Fair values of plan assets | $ 1,527.9 | $ 1,421.4 |
Cash and cash equivalents |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 41.0 | 71.7 |
Equity and debt securities |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 115.5 | 153.4 |
Diversified and equity funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 192.8 | 152.0 |
Fixed Income Funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 95.7 | 55.3 |
Hedge Funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets |  | 15.0 |
Common collective funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 897.0 | 951.0 |
Assets held by Insurance Company |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 26.8 | 30.0 |
Derivative instruments |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 159.1 | (7.0) |
Level 1 |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 342.9 | 276.5 |
Level 1 | Cash and cash equivalents |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 26.4 | 37.1 |
Level 1 | Equity and debt securities |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 115.5 | 153.4 |
Level 1 | Diversified and equity funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 179.0 | 82.4 |
Level 1 | Fixed Income Funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 22.0 | 3.6 |
Level 2 |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 261.2 | 148.9 |
Level 2 | Cash and cash equivalents |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 14.6 | 34.6 |
Level 2 | Diversified and equity funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 13.8 | 69.6 |
Level 2 | Fixed Income Funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 73.7 | 51.7 |
Level 2 | Derivative instruments |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 159.1 | (7.0) |
Level 3 |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 26.8 | 45.0 | $ 45.3
Level 3 | Hedge Funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets |  | 15.0 |
Level 3 | Assets held by Insurance Company |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 26.8 | 30.0 |
Investments measured at NAV |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | 897.0 | 951.0 |
Investments measured at NAV | Common collective funds |  |  |
Post-retirement plan |  |  |
Fair values of plan assets | $ 897.0 | $ 951.0 |

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