FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Assets
Derivative assets	$ 7.2	$ 18.0
Liabilities
Derivative liabilities	70.6	106.4
Fair value measurements on a recurring basis
Assets
Derivative assets	7.2	18.0
Investments held in rabbi trust	41.3	51.7
Liabilities
Derivative liabilities	70.6	106.4
Fair value measurements on a recurring basis | Level 1
Assets
Derivative assets	1.5	2.2
Investments held in rabbi trust	41.3	51.7
Liabilities
Derivative liabilities	51.4	70.1
Fair value measurements on a recurring basis | Level 2
Assets
Derivative assets	3.1	8.6
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities	19.2	36.3
Fair value measurements on a recurring basis | Level 3
Assets
Derivative assets	2.6	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	4.4	10.5
Liabilities
Derivative liabilities	52.6	86.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Derivative assets	1.5	2.2
Liabilities
Derivative liabilities	51.4	70.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Derivative assets	2.9	8.3
Liabilities
Derivative liabilities	1.2	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	2.6	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	2.6	7.2
Fair value measurements on a recurring basis | Coal contracts
Assets
Derivative assets	0.2	0.3
Liabilities
Derivative liabilities	18.0	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	18.0	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