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FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions | Mar. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Assets |  |
Derivative assets | $ 5.8 | $ 6.9
Liabilities |  |
Derivative liabilities | 32.8 | 28.9
Fair value measurements on a recurring basis |  |
Assets |  |
Derivative assets | 5.8 | 6.9
Investments held in rabbi trust | 54.9 | 85.3
Liabilities |  |
Derivative liabilities | 32.8 | 28.9
Fair value measurements on a recurring basis | Level 1 |  |
Assets |  |
Derivative assets | 3.9 | 1.4
Investments held in rabbi trust | 54.9 | 85.3
Liabilities |  |
Derivative liabilities | 22.7 | 21.4
Fair value measurements on a recurring basis | Level 2 |  |
Assets |  |
Derivative assets | 1.0 | 2.4
Investments held in rabbi trust | 0.0 | 0.0
Liabilities |  |
Derivative liabilities | 10.1 | 7.5
Fair value measurements on a recurring basis | Level 3 |  |
Assets |  |
Derivative assets | 0.9 | 3.1
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 | 4.6 | 3.4
Liabilities |  |
Derivative liabilities | 22.7 | 22.7
Fair value measurements on a recurring basis | Natural gas contracts | Level 1 |  |
Assets |  |
Derivative assets | 3.9 | 1.4
Liabilities |  |
Derivative liabilities | 22.7 | 21.4
Fair value measurements on a recurring basis | Natural gas contracts | Level 2 |  |
Assets |  |
Derivative assets | 0.7 | 2.0
Liabilities |  |
Derivative liabilities | 0.0 | 1.3
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 | 3.1
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 | 3.1
Fair value measurements on a recurring basis | Coal contracts |  |
Assets |  |
Derivative assets | 0.3 | 0.4
Liabilities |  |
Derivative liabilities | 0.1 | 0.2
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 | 0.3 | 0.4
Liabilities |  |
Derivative liabilities | 0.1 | 0.2
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 | 10.0 | 6.0
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 | 10.0 | 6.0
Fair value measurements on a recurring basis | Interest rate swaps | Level 3 |  |
Liabilities |  |
Derivative liabilities | $ 0.0 | $ 0.0

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