DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) $ in Millions	Sep. 30, 2021 USD ($) Instruments	Dec. 31, 2020 USD ($)
Derivative assets
Current derivative assets	$ 53.5	$ 3.1
Long-term derivative assets	3.7	0.2
Total derivative assets	57.2	3.3
Derivative liabilities
Current derivative liabilities	0.4	2.0
Long-term derivative liabilities	0.0	0.4
Total derivative liabilities	0.4	2.4
Natural gas contracts
Derivative assets
Current derivative assets	37.3	1.7
Long-term derivative assets	2.4	0.0
Derivative liabilities
Current derivative liabilities	0.4	1.7
Long-term derivative liabilities	0.0	0.2
FTRs
Derivative assets
Current derivative assets	2.1	1.2
Derivative liabilities
Current derivative liabilities	0.0	0.0
Coal contracts
Derivative assets
Current derivative assets	14.1	0.2
Long-term derivative assets	1.3	0.2
Derivative liabilities
Current derivative liabilities	0.0	0.3
Long-term derivative liabilities	$ 0.0	$ 0.2
Derivatives designated as hedging instruments
Derivative instruments
Number of derivative instruments | Instruments	0