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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Amount of restricted investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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-Name Accounting Standards Codification
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