DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) $ in Millions	Dec. 31, 2023 USD ($) Instruments	Dec. 31, 2022 USD ($) Instruments
Derivative assets
Current derivative assets	$ 4.2	$ 7.2
Long-term derivative assets	0.0	0.4
Total derivative assets	$ 4.2	$ 7.6
Current derivative assets balance sheet location	Other	Other
Long-term derivative assets balance sheet location	Other	Other
Derivative liabilities
Current derivative liabilities	$ 8.1	$ 16.3
Long-term derivative liabilities	0.8	0.6
Total derivative liabilities	$ 8.9	$ 16.9
Current derivative liabilities balance sheet location	Other	Other
Long-term derivative liabilities balance sheet location	Other	Other
Natural gas contracts
Derivative assets
Current derivative assets	$ 1.9	$ 1.7
Long-term derivative assets	0.0	0.0
Derivative liabilities
Current derivative liabilities	7.4	16.3
Long-term derivative liabilities	0.5	0.6
FTRs
Derivative assets
Current derivative assets	2.0	4.1
Derivative liabilities
Current derivative liabilities	0.0	0.0
Coal contracts
Derivative assets
Current derivative assets	0.3	1.4
Long-term derivative assets	0.0	0.4
Derivative liabilities
Current derivative liabilities	0.7	0.0
Long-term derivative liabilities	$ 0.3	$ 0.0
Derivatives designated as hedging instruments
Derivative instruments
Number of derivative instruments | Instruments	0	0