FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Assets
Derivative asset	$ 15.8	$ 15.9
Liabilities
Derivative liability	31.1	7.9
Fair value measurements on a recurring basis
Assets
Derivative asset	15.8	15.9
Investments held in rabbi trust	77.1	65.0
Liabilities
Derivative liability	31.1	7.9
Fair value measurements on a recurring basis \| Level 1
Assets
Derivative asset	2.6	6.3
Investments held in rabbi trust	77.1	65.0
Liabilities
Derivative liability	23.8	4.7
Fair value measurements on a recurring basis \| Level 2
Assets
Derivative asset	2.8	2.2
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liability	7.3	3.2
Fair value measurements on a recurring basis \| Level 3
Assets
Derivative asset	10.4	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	4.6	8.1
Liabilities
Derivative liability	24.3	5.5
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 1
Assets
Derivative asset	2.6	6.3
Liabilities
Derivative liability	23.8	4.7
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 2
Assets
Derivative asset	2.0	1.8
Liabilities
Derivative liability	0.5	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	10.4	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	10.4	7.4
Fair value measurements on a recurring basis \| Coal contracts
Assets
Derivative asset	0.8	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	0.8	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	6.7	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	6.7	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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