FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Assets
Derivative assets	$ 219.2	$ 120.0
Liabilities
Derivative liabilities	49.6	15.1
Fair value measurements on a recurring basis
Assets
Derivative assets	219.2	120.0
Investments held in rabbi trust	47.4	79.6
Fair value measurements on a recurring basis \| Level 1
Assets
Derivative assets	109.1	46.4
Investments held in rabbi trust	47.4	79.6
Fair value measurements on a recurring basis \| Level 2
Assets
Derivative assets	98.1	71.2
Investments held in rabbi trust	0.0	0.0
Fair value measurements on a recurring basis \| Level 3
Assets
Derivative assets	12.0	2.4
Investments held in rabbi trust	0.0	0.0
Fair value measurements on a recurring basis \| Natural gas contracts
Assets
Derivative assets	141.4	64.6
Liabilities
Derivative liabilities	49.6	15.1
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 1
Assets
Derivative assets	109.1	46.4
Liabilities
Derivative liabilities	24.3	8.4
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 2
Assets
Derivative assets	32.3	18.2
Liabilities
Derivative liabilities	25.3	6.7
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 and TCRs
Assets
Derivative assets	12.0	2.4
Fair value measurements on a recurring basis \| FTRs and TCRs \| Level 1
Assets
Derivative assets	0.0	0.0
Fair value measurements on a recurring basis \| FTRs and TCRs \| Level 2
Assets
Derivative assets	0.0	0.0
Fair value measurements on a recurring basis \| FTRs and TCRs \| Level 3
Assets
Derivative assets	12.0	2.4
Fair value measurements on a recurring basis \| Coal contracts
Assets
Derivative assets	65.8	53.0
Fair value measurements on a recurring basis \| Coal contracts \| Level 1
Assets
Derivative assets	0.0	0.0
Fair value measurements on a recurring basis \| Coal contracts \| Level 2
Assets
Derivative assets	65.8	53.0
Fair value measurements on a recurring basis \| Coal contracts \| Level 3
Assets
Derivative assets	$ 0.0	$ 0.0

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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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