FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets
Derivative asset	$ 23.0	$ 12.4
Liabilities
Derivative liability	4.9	11.6
Fair value measurements on a recurring basis
Assets
Derivative asset	23.0	12.4
Investments held in rabbi trust	115.3	120.7
Liabilities
Derivative liability		11.6
Fair value measurements on a recurring basis \| Level 1
Assets
Derivative asset	9.6	3.0
Investments held in rabbi trust	115.3	120.7
Liabilities
Derivative liability		7.0
Fair value measurements on a recurring basis \| Level 2
Assets
Derivative asset	1.9	5.0
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liability		4.6
Fair value measurements on a recurring basis \| Level 3
Assets
Derivative asset	11.5	4.4
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liability		0.0
Fair value measurements on a recurring basis \| Natural gas contracts
Assets
Derivative asset	9.9	5.7
Liabilities
Derivative liability	4.9	10.8
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 1
Assets
Derivative asset	9.6	1.8
Liabilities
Derivative liability	3.6	7.0
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 2
Assets
Derivative asset	0.3	3.9
Liabilities
Derivative liability	1.3	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		1.2
Fair value measurements on a recurring basis \| Petroleum products contracts \| Level 1
Assets
Derivative asset		1.2
Fair value measurements on a recurring basis \| Petroleum products contracts \| Level 2
Assets
Derivative asset		0.0
Fair value measurements on a recurring basis \| Petroleum products contracts \| Level 3
Assets
Derivative asset		0.0
Fair value measurements on a recurring basis \| FTRs
Assets
Derivative asset	11.5	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	11.5	4.4
Fair value measurements on a recurring basis \| Coal contracts
Assets
Derivative asset	0.8	1.1
Liabilities
Derivative liability		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
Fair value measurements on a recurring basis \| Coal contracts \| Level 2
Assets
Derivative asset	0.8	1.1
Liabilities
Derivative liability		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
Fair value measurements on a recurring basis \| Interest rate swaps
Assets
Derivative asset	0.8
Fair value measurements on a recurring basis \| Interest rate swaps \| Level 1
Assets
Derivative asset	0.0
Fair value measurements on a recurring basis \| Interest rate swaps \| Level 2
Assets
Derivative asset	0.8
Fair value measurements on a recurring basis \| Interest rate swaps \| Level 3
Assets
Derivative asset	$ 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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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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