FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Assets
Derivative assets	$ 24.8	$ 74.8
Liabilities
Derivative liabilities	108.5	96.6
Fair value measurements on a recurring basis
Assets
Derivative assets	24.8	74.8
Investments held in rabbi trust	46.2	50.9
Liabilities
Derivative liabilities	108.5
Fair value measurements on a recurring basis | Level 1
Assets
Derivative assets	2.4	16.3
Investments held in rabbi trust	46.2	50.9
Liabilities
Derivative liabilities	70.7
Fair value measurements on a recurring basis | Level 2
Assets
Derivative assets	11.1	50.7
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities	37.8
Fair value measurements on a recurring basis | Level 3
Assets
Derivative assets	11.3	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	12.7	32.5
Liabilities
Derivative liabilities	83.4	96.6
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Derivative assets	2.4	16.3
Liabilities
Derivative liabilities	70.7	81.4
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Derivative assets	10.3	16.2
Liabilities
Derivative liabilities	12.7	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	11.3	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	11.3	7.8
Fair value measurements on a recurring basis | Coal contracts
Assets
Derivative assets	0.8	34.5
Liabilities
Derivative liabilities	25.1
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	0.8	34.5
Liabilities
Derivative liabilities	25.1
Fair value measurements on a recurring basis | Coal contracts | Level 3
Assets
Derivative assets	0.0	$ 0.0
Liabilities
Derivative liabilities	$ 0.0