FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Assets
Derivative assets	$ 219.1	$ 120.0
Liabilities
Derivative liabilities	1.4	15.1
Fair value measurements on a recurring basis
Assets
Derivative assets	219.1	120.0
Investments held in rabbi trust	59.5	79.6
Fair value measurements on a recurring basis \| Level 1
Assets
Derivative assets	176.9	46.4
Investments held in rabbi trust	59.5	79.6
Fair value measurements on a recurring basis \| Level 2
Assets
Derivative assets	41.2	71.2
Investments held in rabbi trust	0.0	0.0
Fair value measurements on a recurring basis \| Level 3
Assets
Derivative assets	1.0	2.4
Investments held in rabbi trust	0.0	0.0
Fair value measurements on a recurring basis \| Natural gas contracts
Assets
Derivative assets	179.9	64.6
Liabilities
Derivative liabilities	1.4	15.1
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 1
Assets
Derivative assets	176.9	46.4
Liabilities
Derivative liabilities	0.0	8.4
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 2
Assets
Derivative assets	3.0	18.2
Liabilities
Derivative liabilities	1.4	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
Assets
Derivative assets	1.0	2.4
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	1.0	2.4
Fair value measurements on a recurring basis \| Coal contracts
Assets
Derivative assets	38.2	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	38.2	53.0
Fair value measurements on a recurring basis \| Coal contracts \| Level 3
Assets
Derivative assets	$ 0.0	$ 0.0

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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