DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) $ in Millions	Jun. 30, 2024 USD ($) Instruments	Dec. 31, 2023 USD ($) Instruments
Derivative assets
Current derivative assets	$ 9.7	$ 4.2
Long-term derivative assets	0.0	0.0
Total derivative assets	$ 9.7	$ 4.2
Current derivative assets balance sheet location	Other	Other
Long-term derivative assets balance sheet location	Other	Other
Derivative liabilities
Current derivative liabilities	$ 4.0	$ 8.1
Long-term derivative liabilities	0.3	0.8
Derivative liabilities	$ 4.3	$ 8.9
Current derivative liability balance sheet location	Other	Other
Long-term derivative liabilities balance sheet location	Other	Other
Natural gas contracts
Derivative assets
Current derivative assets	$ 2.7	$ 1.9
Long-term derivative assets	0.0	0.0
Derivative liabilities
Current derivative liabilities	3.2	7.4
Long-term derivative liabilities	0.1	0.5
FTRs
Derivative assets
Current derivative assets	6.8	2.0
Derivative liabilities
Current derivative liabilities	0.0	0.0
Coal contracts
Derivative assets
Current derivative assets	0.2	0.3
Long-term derivative assets	0.0	0.0
Derivative liabilities
Current derivative liabilities	0.8	0.7
Long-term derivative liabilities	$ 0.2	$ 0.3
Derivatives designated as hedging instruments
Derivative instruments
Number of derivative instruments | Instruments	0	0