DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) $ in Millions	Dec. 31, 2022 USD ($) Instruments	Dec. 31, 2021 USD ($) Instruments
Derivative assets
Other current derivative assets	$ 7.2	$ 22.2
Other long-term derivative assets	0.4	4.0
Derivative assets	$ 7.6	$ 26.2
Long-term derivative assets balance sheet location	Other	Other
Derivative liabilities
Other current derivative liabilities	$ 16.3	$ 1.5
Other long-term derivative liabilities	0.6	0.1
Derivative liabilities	$ 16.9	$ 1.6
Current derivative liabilities balance sheet location	Other	Other
Long-term derivative liabilities balance sheet location	Other	Other
Natural gas contracts
Derivative assets
Other current derivative assets	$ 1.7	$ 9.2
Other long-term derivative assets	0.0	0.3
Derivative liabilities
Other current derivative liabilities	16.3	1.5
Other long-term derivative liabilities	0.6	0.1
FTRs
Derivative assets
Other current derivative assets	4.1	1.4
Derivative liabilities
Other current derivative liabilities	0.0	0.0
Coal contracts
Derivative assets
Other current derivative assets	1.4	11.6
Other long-term derivative assets	0.4	3.7
Derivative liabilities
Other current derivative liabilities	0.0	0.0
Other long-term derivative liabilities	$ 0.0	$ 0.0
Derivatives designated as hedging instruments
Derivative instruments
Number of derivative instruments | Instruments	0	0