FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Assets
Derivative assets	$ 28.5	$ 74.8
Liabilities
Derivative liabilities	140.4	96.6
Fair value measurements on a recurring basis
Assets
Derivative assets	28.5	74.8
Investments held in rabbi trust	47.8	50.9
Liabilities
Derivative liabilities	140.4
Fair value measurements on a recurring basis | Level 1
Assets
Derivative assets	2.8	16.3
Investments held in rabbi trust	47.8	50.9
Liabilities
Derivative liabilities	100.3
Fair value measurements on a recurring basis | Level 2
Assets
Derivative assets	8.9	50.7
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities	40.1
Fair value measurements on a recurring basis | Level 3
Assets
Derivative assets	16.8	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	10.5	32.5
Liabilities
Derivative liabilities	112.3	96.6
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Derivative assets	2.8	16.3
Liabilities
Derivative liabilities	100.3	81.4
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Derivative assets	7.7	16.2
Liabilities
Derivative liabilities	12.0	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 and TCRs
Assets
Derivative assets	16.8	7.8
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	16.8	7.8
Fair value measurements on a recurring basis | Coal contracts
Assets
Derivative assets	1.2	34.5
Liabilities
Derivative liabilities	28.1
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	1.2	34.5
Liabilities
Derivative liabilities	28.1
Fair value measurements on a recurring basis | Coal contracts | Level 3
Assets
Derivative assets	0.0	$ 0.0
Liabilities
Derivative liabilities	$ 0.0