FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Assets
Derivative assets	$ 7.7	$ 74.8
Liabilities
Derivative liabilities	169.8	96.6
Fair value measurements on a recurring basis
Assets
Derivative assets	7.7	74.8
Investments held in rabbi trust	44.0	50.9
Liabilities
Derivative liabilities	169.8
Fair value measurements on a recurring basis \| Level 1
Assets
Derivative assets	0.5	16.3
Investments held in rabbi trust	44.0	50.9
Liabilities
Derivative liabilities	153.5
Fair value measurements on a recurring basis \| Level 2
Assets
Derivative assets	4.2	50.7
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities	16.3
Fair value measurements on a recurring basis \| Level 3
Assets
Derivative assets	3.0	7.8
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities	0.0
Fair value measurements on a recurring basis \| Natural gas contracts
Assets
Derivative assets	3.0	32.5
Liabilities
Derivative liabilities	155.1	96.6
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 1
Assets
Derivative assets	0.5	16.3
Liabilities
Derivative liabilities	153.5	81.4
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 2
Assets
Derivative assets	2.5	16.2
Liabilities
Derivative liabilities	1.6	15.2
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 3
Assets
Derivative assets	0.0	0.0
Liabilities
Derivative liabilities	0.0	0.0
Fair value measurements on a recurring basis \| FTRs
Assets
Derivative assets	3.0	7.8
Fair value measurements on a recurring basis \| FTRs \| Level 1
Assets
Derivative assets	0.0	0.0
Fair value measurements on a recurring basis \| FTRs \| Level 2
Assets
Derivative assets	0.0	0.0
Fair value measurements on a recurring basis \| FTRs \| Level 3
Assets
Derivative assets	3.0	7.8
Fair value measurements on a recurring basis \| Coal contracts
Assets
Derivative assets	1.7	34.5
Liabilities
Derivative liabilities	14.7
Fair value measurements on a recurring basis \| Coal contracts \| Level 1
Assets
Derivative assets	0.0	0.0
Liabilities
Derivative liabilities	0.0
Fair value measurements on a recurring basis \| Coal contracts \| Level 2
Assets
Derivative assets	1.7	34.5
Liabilities
Derivative liabilities	14.7
Fair value measurements on a recurring basis \| Coal contracts \| Level 3
Assets
Derivative assets	0.0	$ 0.0
Liabilities
Derivative liabilities	$ 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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Section 50
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-Publisher FASB
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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