DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) $ in Millions	Sep. 30, 2023 USD ($) Instruments	Dec. 31, 2022 USD ($) Instruments
Derivative assets
Current derivative assets	$ 24.4	$ 59.2
Long-term derivative assets	0.4	15.6
Total derivative assets	$ 24.8	$ 74.8
Current derivative assets balance sheet location	Other	Other
Long-term derivative assets balance sheet location	Other Assets, Noncurrent	Other Assets, Noncurrent
Derivative liabilities
Current derivative liabilities	$ 94.9	$ 88.2
Long-term derivative liabilities	13.6	8.4
Derivative liabilities	$ 108.5	$ 96.6
Current derivative liabilities balance sheet location	Other Liabilities, Current	Other Liabilities, Current
Long-term derivative liabilities balance sheet location	Other noncurrent liabilities	Other noncurrent liabilities
Natural gas contracts
Derivative assets
Current derivative assets	$ 12.5	$ 32.5
Long-term derivative assets	0.2	0.0
Derivative liabilities
Current derivative liabilities	81.1	88.2
Long-term derivative liabilities	2.3	8.4
FTRs and TCRs
Derivative assets
Current derivative assets	11.3	7.8
Derivative liabilities
Current derivative liabilities	0.0	0.0
Coal contracts
Derivative assets
Current derivative assets	0.6	18.9
Long-term derivative assets	0.2	15.6
Derivative liabilities
Current derivative liabilities	13.8	0.0
Long-term derivative liabilities	$ 11.3	$ 0.0
Designated as hedging instrument
Derivative instruments
Number of derivative instruments | Instruments	0	0