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FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Assets |  |
Derivative assets | $ 17.4 | $ 17.9
Liabilities |  |
Derivative liabilities | 9.5 | 22.1
Fair value measurements on a recurring basis |  |
Assets |  |
Derivative assets | 17.4 | 17.9
Investments held in rabbi trust | 71.6 | 79.6
Liabilities |  |
Derivative liabilities | 9.5 | 22.1
Fair value measurements on a recurring basis | Level 1 |  |
Assets |  |
Derivative assets | 12.3 | 11.7
Investments held in rabbi trust | 71.6 | 79.6
Liabilities |  |
Derivative liabilities | 3.1 | 7.7
Fair value measurements on a recurring basis | Level 2 |  |
Assets |  |
Derivative assets | 4.2 | 3.8
Investments held in rabbi trust | 0.0 | 0.0
Liabilities |  |
Derivative liabilities | 6.4 | 14.4
Fair value measurements on a recurring basis | Level 3 |  |
Assets |  |
Derivative assets | 0.9 | 2.4
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 | 13.9 | 13.7
Liabilities |  |
Derivative liabilities | 4.1 | 14.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 1 |  |
Assets |  |
Derivative assets | 12.3 | 11.7
Liabilities |  |
Derivative liabilities | 3.1 | 7.7
Fair value measurements on a recurring basis | Natural gas contracts | Level 2 |  |
Assets |  |
Derivative assets | 1.6 | 2.0
Liabilities |  |
Derivative liabilities | 1.0 | 6.4
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 |  |
Assets |  |
Derivative assets | 0.9 | 2.4
Fair value measurements on a recurring basis | FTRs | Level 1 |  |
Assets |  |
Derivative assets | 0.0 | 0.0
Fair value measurements on a recurring basis | FTRs | Level 2 |  |
Assets |  |
Derivative assets | 0.0 | 0.0
Fair value measurements on a recurring basis | FTRs | Level 3 |  |
Assets |  |
Derivative assets | 0.9 | 2.4
Fair value measurements on a recurring basis | Coal contracts |  |
Assets |  |
Derivative assets | 2.6 | 1.8
Liabilities |  |
Derivative liabilities | 0.3 | 1.2
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 | 2.6 | 1.8
Liabilities |  |
Derivative liabilities | 0.3 | 1.2
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
Fair value measurements on a recurring basis | Interest rate swaps |  |
Liabilities |  |
Derivative liabilities | 5.1 | 6.8
Fair value measurements on a recurring basis | Interest rate swaps | Level 1 |  |
Liabilities |  |
Derivative liabilities | 0.0 | 0.0
Fair value measurements on a recurring basis | Interest rate swaps | Level 2 |  |
Liabilities |  |
Derivative liabilities | 5.1 | 6.8
Fair value measurements on a recurring basis | Interest rate swaps | Level 3 |  |
Liabilities |  |
Derivative liabilities | $ 0.0 | $ 0.0

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