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FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017
Assets |  |  |
Derivative asset | $ 5.4 |  | $ 12.4
Liabilities |  |  |
Derivative liability | 5.1 |  | 11.6
Fair value measurements on a recurring basis |  |  |
Assets |  |  |
Derivative asset | 5.4 |  | 12.4
Investments held in rabbi trust | 103.6 |  | 120.7
Liabilities |  |  |
Derivative liability | 5.1 |  | 11.6
Net Unrealized Gain (Loss) on Investments |  |  |
Net unrealized gains (losses) included in earnings related to investments held in rabbi trust | (3.1) | $ 5.2 |
Fair value measurements on a recurring basis | Level 1 |  |  |
Assets |  |  |
Derivative asset | 2.3 |  | 3.0
Investments held in rabbi trust | 103.6 |  | 120.7
Liabilities |  |  |
Derivative liability | 3.4 |  | 7.0
Fair value measurements on a recurring basis | Level 2 |  |  |
Assets |  |  |
Derivative asset | 1.6 |  | 5.0
Investments held in rabbi trust | 0.0 |  | 0.0
Liabilities |  |  |
Derivative liability | 1.7 |  | 4.6
Fair value measurements on a recurring basis | Level 3 |  |  |
Assets |  |  |
Derivative asset | 1.5 |  | 4.4
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 | 2.2 |  | 5.7
Liabilities |  |  |
Derivative liability | 4.8 |  | 10.8
Fair value measurements on a recurring basis | Natural gas contracts | Level 1 |  |  |
Assets |  |  |
Derivative asset | 1.7 |  | 1.8
Liabilities |  |  |
Derivative liability | 3.4 |  | 7.0
Fair value measurements on a recurring basis | Natural gas contracts | Level 2 |  |  |
Assets |  |  |
Derivative asset | 0.5 |  | 3.9
Liabilities |  |  |
Derivative liability | 1.4 |  | 3.8
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 | Petroleum products contracts |  |  |
Assets |  |  |
Derivative asset | 0.6 |  | 1.2
Fair value measurements on a recurring basis | Petroleum products contracts | Level 1 |  |  |
Assets |  |  |
Derivative asset | 0.6 |  | 1.2
Fair value measurements on a recurring basis | Petroleum products contracts | Level 2 |  |  |
Assets |  |  |
Derivative asset | 0.0 |  | 0.0
Fair value measurements on a recurring basis | Petroleum products contracts | Level 3 |  |  |
Assets |  |  |
Derivative asset | 0.0 |  | 0.0
Fair value measurements on a recurring basis | FTRs |  |  |
Assets |  |  |
Derivative asset | 1.5 |  | 4.4
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 | 1.5 |  | 4.4
Fair value measurements on a recurring basis | Coal contracts |  |  |
Assets |  |  |
Derivative asset | 1.1 |  | 1.1
Liabilities |  |  |
Derivative liability | 0.3 |  | 0.8
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 | 1.1 |  | 1.1
Liabilities |  |  |
Derivative liability | 0.3 |  | 0.8
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

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