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FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Assets |  |
Derivative asset | $ 10.3 | $ 15.9
Liabilities |  |
Derivative liability | 4.7 | 7.9
Fair value measurements on a recurring basis |  |
Assets |  |
Derivative asset | 10.3 | 15.9
Investments held in rabbi trust | 74.0 | 65.0
Liabilities |  |
Derivative liability | 4.7 | 7.9
Fair value measurements on a recurring basis | Level 1 |  |
Assets |  |
Derivative asset | 6.0 | 6.3
Investments held in rabbi trust | 74.0 | 65.0
Liabilities |  |
Derivative liability | 0.9 | 4.7
Fair value measurements on a recurring basis | Level 2 |  |
Assets |  |
Derivative asset | 1.2 | 2.2
Investments held in rabbi trust | 0.0 | 0.0
Liabilities |  |
Derivative liability | 3.8 | 3.2
Fair value measurements on a recurring basis | Level 3 |  |
Assets |  |
Derivative asset | 3.1 | 7.4
Investments held in rabbi trust | 0.0 | 0.0
Liabilities |  |
Derivative liability | 0.0 | 0.0
Fair value measurements on a recurring basis | Natural gas contracts |  |
Assets |  |
Derivative asset | 6.0 | 8.1
Liabilities |  |
Derivative liability | 0.9 | 5.5
Fair value measurements on a recurring basis | Natural gas contracts | Level 1 |  |
Assets |  |
Derivative asset | 6.0 | 6.3
Liabilities |  |
Derivative liability | 0.9 | 4.7
Fair value measurements on a recurring basis | Natural gas contracts | Level 2 |  |
Assets |  |
Derivative asset | 0.0 | 1.8
Liabilities |  |
Derivative liability | 0.0 | 0.8
Fair value measurements on a recurring basis | Natural gas contracts | Level 3 |  |
Assets |  |
Derivative asset | 0.0 | 0.0
Liabilities |  |
Derivative liability | 0.0 | 0.0
Fair value measurements on a recurring basis | FTRs |  |
Assets |  |
Derivative asset | 3.1 | 7.4
Fair value measurements on a recurring basis | FTRs | Level 1 |  |
Assets |  |
Derivative asset | 0.0 | 0.0
Fair value measurements on a recurring basis | FTRs | Level 2 |  |
Assets |  |
Derivative asset | 0.0 | 0.0
Fair value measurements on a recurring basis | FTRs | Level 3 |  |
Assets |  |
Derivative asset | 3.1 | 7.4
Fair value measurements on a recurring basis | Coal contracts |  |
Assets |  |
Derivative asset | 1.2 | 0.4
Liabilities |  |
Derivative liability | 0.1 | 0.1
Fair value measurements on a recurring basis | Coal contracts | Level 1 |  |
Assets |  |
Derivative asset | 0.0 | 0.0
Liabilities |  |
Derivative liability | 0.0 | 0.0
Fair value measurements on a recurring basis | Coal contracts | Level 2 |  |
Assets |  |
Derivative asset | 1.2 | 0.4
Liabilities |  |
Derivative liability | 0.1 | 0.1
Fair value measurements on a recurring basis | Coal contracts | Level 3 |  |
Assets |  |
Derivative asset | 0.0 | 0.0
Liabilities |  |
Derivative liability | 0.0 | 0.0
Fair value measurements on a recurring basis | Interest rate swaps |  |
Liabilities |  |
Derivative liability | 3.7 | 2.3
Fair value measurements on a recurring basis | Interest rate swaps | Level 1 |  |
Liabilities |  |
Derivative liability | 0.0 | 0.0
Fair value measurements on a recurring basis | Interest rate swaps | Level 2 |  |
Liabilities |  |
Derivative liability | 3.7 | 2.3
Fair value measurements on a recurring basis | Interest rate swaps | Level 3 |  |
Liabilities |  |
Derivative liability | $ 0.0 | $ 0.0

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