FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Assets
Derivative assets	$ 34.3	$ 18.0
Liabilities
Derivative liabilities	56.3	106.4
Fair value measurements on a recurring basis
Assets
Derivative assets	34.3	18.0
Investments held in rabbi trust	47.9	51.7
Liabilities
Derivative liabilities	56.3	106.4
Fair value measurements on a recurring basis | Level 1
Assets
Derivative assets	4.0	2.2
Investments held in rabbi trust	47.9	51.7
Liabilities
Derivative liabilities	27.6	70.1
Fair value measurements on a recurring basis | Level 2
Assets
Derivative assets	9.5	8.6
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities	28.7	36.3
Fair value measurements on a recurring basis | Level 3
Assets
Derivative assets	20.8	7.2
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities	0.0	0.0
Fair value measurements on a recurring basis | Natural gas contracts
Assets
Derivative assets	13.3	10.5
Liabilities
Derivative liabilities	40.6	86.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Derivative assets	4.0	2.2
Liabilities
Derivative liabilities	27.6	70.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Derivative assets	9.3	8.3
Liabilities
Derivative liabilities	13.0	16.0
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	20.8	7.2
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	20.8	7.2
Fair value measurements on a recurring basis | Coal contracts
Assets
Derivative assets	0.2	0.3
Liabilities
Derivative liabilities	15.7	20.3
Fair value measurements on a recurring basis | Coal contracts | Level 1
Assets
Derivative assets	0.0	0.0
Liabilities
Derivative liabilities	0.0	0.0
Fair value measurements on a recurring basis | Coal contracts | Level 2
Assets
Derivative assets	0.2	0.3
Liabilities
Derivative liabilities	15.7	20.3
Fair value measurements on a recurring basis | Coal contracts | Level 3
Assets
Derivative assets	0.0	0.0
Liabilities
Derivative liabilities	$ 0.0	$ 0.0