FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Assets
Derivative assets	$ 56.4	$ 41.1
Liabilities
Derivative liabilities	15.8	13.9
Fair value measurements on a recurring basis
Assets
Derivative assets	56.4	41.1
Investments held in rabbi trust	37.9	52.1
Fair value measurements on a recurring basis | Level 1
Assets
Derivative assets	19.2	19.6
Investments held in rabbi trust	37.9	52.1
Fair value measurements on a recurring basis | Level 2
Assets
Derivative assets	18.2	13.7
Investments held in rabbi trust	0.0	0.0
Fair value measurements on a recurring basis | Level 3
Assets
Derivative assets	19.0	7.8
Investments held in rabbi trust	0.0	0.0
Fair value measurements on a recurring basis | Natural gas contracts
Assets
Derivative assets	37.4	33.3
Liabilities
Derivative liabilities	15.8	13.9
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Derivative assets	19.2	19.6
Liabilities
Derivative liabilities	6.5	7.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Derivative assets	18.2	13.7
Liabilities
Derivative liabilities	9.3	6.8
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	19.0	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	$ 19.0	$ 7.8