DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) $ in Millions	Dec. 31, 2021 USD ($) Instruments	Dec. 31, 2020 USD ($)
Derivative assets
Derivative assets	$ 22.2	$ 3.1
Other long-term derivative assets	4.0	0.2
Derivative assets	26.2	3.3
Derivative liabilities
Other current derivative liabilities	1.5	2.0
Other long-term derivative liabilities	0.1	0.4
Derivative liabilities	1.6	2.4
Natural gas contracts
Derivative assets
Derivative assets	9.2	1.7
Other long-term derivative assets	0.3	0.0
Derivative liabilities
Other current derivative liabilities	1.5	1.7
Other long-term derivative liabilities	0.1	0.2
FTRs
Derivative assets
Derivative assets	1.4	1.2
Derivative liabilities
Other current derivative liabilities	0.0	0.0
Coal contracts
Derivative assets
Derivative assets	11.6	0.2
Other long-term derivative assets	3.7	0.2
Derivative liabilities
Other current derivative liabilities	0.0	0.3
Other long-term derivative liabilities	$ 0.0	$ 0.2
Derivatives designated as hedging instruments
Derivative instruments
Number of derivative instruments | Instruments	0