FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Assets
Derivative assets	$ 9.7	$ 4.2
Liabilities
Derivative liabilities	4.3	8.9
Fair value measurements on a recurring basis
Assets
Derivative assets	9.7	4.2
Liabilities
Derivative liabilities	4.3	8.9
Fair value measurements on a recurring basis | Level 1
Assets
Derivative assets	0.8	0.6
Liabilities
Derivative liabilities	2.7	7.4
Fair value measurements on a recurring basis | Level 2
Assets
Derivative assets	2.1	1.6
Liabilities
Derivative liabilities	1.6	1.5
Fair value measurements on a recurring basis | Level 3
Assets
Derivative assets	6.8	2.0
Liabilities
Derivative liabilities	0.0	0.0
Fair value measurements on a recurring basis | Natural gas contracts
Assets
Derivative assets	2.7	1.9
Liabilities
Derivative liabilities	3.3	7.9
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Derivative assets	0.8	0.6
Liabilities
Derivative liabilities	2.7	7.4
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Derivative assets	1.9	1.3
Liabilities
Derivative liabilities	0.6	0.5
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
Assets
Derivative assets	6.8	2.0
Fair value measurements on a recurring basis | FTRs | Level 1
Assets
Derivative assets	0.0	0.0
Fair value measurements on a recurring basis | FTRs | Level 2
Assets
Derivative assets	0.0	0.0
Fair value measurements on a recurring basis | FTRs | Level 3
Assets
Derivative assets	6.8	2.0
Fair value measurements on a recurring basis | Coal contracts
Assets
Derivative assets	0.2	0.3
Liabilities
Derivative liabilities	1.0	1.0
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	1.0	1.0
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