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FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions | Mar. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Assets |  |
Derivative assets | $ 7.2 | $ 18.0
Liabilities |  |
Derivative liabilities | 70.6 | 106.4
Fair value measurements on a recurring basis |  |
Assets |  |
Derivative assets | 7.2 | 18.0
Investments held in rabbi trust | 41.3 | 51.7
Liabilities |  |
Derivative liabilities | 70.6 | 106.4
Fair value measurements on a recurring basis | Level 1 |  |
Assets |  |
Derivative assets | 1.5 | 2.2
Investments held in rabbi trust | 41.3 | 51.7
Liabilities |  |
Derivative liabilities | 51.4 | 70.1
Fair value measurements on a recurring basis | Level 2 |  |
Assets |  |
Derivative assets | 3.1 | 8.6
Investments held in rabbi trust | 0.0 | 0.0
Liabilities |  |
Derivative liabilities | 19.2 | 36.3
Fair value measurements on a recurring basis | Level 3 |  |
Assets |  |
Derivative assets | 2.6 | 7.2
Investments held in rabbi trust | 0.0 | 0.0
Liabilities |  |
Derivative liabilities | 0.0 | 0.0
Fair value measurements on a recurring basis | Natural gas contracts |  |
Assets |  |
Derivative assets | 4.4 | 10.5
Liabilities |  |
Derivative liabilities | 52.6 | 86.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 1 |  |
Assets |  |
Derivative assets | 1.5 | 2.2
Liabilities |  |
Derivative liabilities | 51.4 | 70.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 2 |  |
Assets |  |
Derivative assets | 2.9 | 8.3
Liabilities |  |
Derivative liabilities | 1.2 | 16.0
Fair value measurements on a recurring basis | Natural gas contracts | Level 3 |  |
Assets |  |
Derivative assets | 0.0 | 0.0
Liabilities |  |
Derivative liabilities | 0.0 | 0.0
Fair value measurements on a recurring basis | FTRs and TCRs |  |
Assets |  |
Derivative assets | 2.6 | 7.2
Fair value measurements on a recurring basis | FTRs and TCRs | Level 1 |  |
Assets |  |
Derivative assets | 0.0 | 0.0
Fair value measurements on a recurring basis | FTRs and TCRs | Level 2 |  |
Assets |  |
Derivative assets | 0.0 | 0.0
Fair value measurements on a recurring basis | FTRs and TCRs | Level 3 |  |
Assets |  |
Derivative assets | 2.6 | 7.2
Fair value measurements on a recurring basis | Coal contracts |  |
Assets |  |
Derivative assets | 0.2 | 0.3
Liabilities |  |
Derivative liabilities | 18.0 | 20.3
Fair value measurements on a recurring basis | Coal contracts | Level 1 |  |
Assets |  |
Derivative assets | 0.0 | 0.0
Liabilities |  |
Derivative liabilities | 0.0 | 0.0
Fair value measurements on a recurring basis | Coal contracts | Level 2 |  |
Assets |  |
Derivative assets | 0.2 | 0.3
Liabilities |  |
Derivative liabilities | 18.0 | 20.3
Fair value measurements on a recurring basis | Coal contracts | Level 3 |  |
Assets |  |
Derivative assets | 0.0 | 0.0
Liabilities |  |
Derivative liabilities | $ 0.0 | $ 0.0

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