FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Assets
Derivative asset	$ 23.7	$ 12.4
Liabilities
Derivative liability	2.6	11.6
Fair value measurements on a recurring basis
Assets
Derivative asset	23.7	12.4
Investments held in rabbi trust	107.6	120.7
Liabilities
Derivative liability	2.6	11.6
Fair value measurements on a recurring basis \| Level 1
Assets
Derivative asset	5.8	3.0
Investments held in rabbi trust	107.6	120.7
Liabilities
Derivative liability	2.1	7.0
Fair value measurements on a recurring basis \| Level 2
Assets
Derivative asset	1.2	5.0
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liability	0.5	4.6
Fair value measurements on a recurring basis \| Level 3
Assets
Derivative asset	16.7	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	6.0	5.7
Liabilities
Derivative liability	2.3	10.8
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 1
Assets
Derivative asset	5.4	1.8
Liabilities
Derivative liability	2.1	7.0
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 2
Assets
Derivative asset	0.6	3.9
Liabilities
Derivative liability	0.2	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	0.4	1.2
Fair value measurements on a recurring basis \| Petroleum products contracts \| Level 1
Assets
Derivative asset	0.4	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	16.7	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	16.7	4.4
Fair value measurements on a recurring basis \| Coal contracts
Assets
Derivative asset	0.6	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	0.6	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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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and elected not to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and elected not to be offset.

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Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Topic 320
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