DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) $ in Millions	Sep. 30, 2021 USD ($) Instruments	Dec. 31, 2020 USD ($)
Derivative assets
Current derivative assets	$ 85.5	$ 6.2
Long-term derivative assets	8.9	0.1
Total derivative assets	94.4	6.3
Derivative liabilities
Current derivative liabilities	0.4	3.7
Long-term derivative liabilities	0.0	0.4
Total derivative liabilities	0.4	4.1
Natural gas contracts
Derivative assets
Current derivative assets	59.3	3.7
Long-term derivative assets	4.7	0.1
Derivative liabilities
Current derivative liabilities	0.4	3.2
Long-term derivative liabilities	0.0	0.3
FTRs
Derivative assets
Current derivative assets	1.6	1.1
Derivative liabilities
Current derivative liabilities	0.0	0.0
Coal contracts
Derivative assets
Current derivative assets	24.6	1.4
Long-term derivative assets	4.2	0.0
Derivative liabilities
Current derivative liabilities	0.0	0.5
Long-term derivative liabilities	$ 0.0	$ 0.1
Derivatives designated as hedging instruments
Derivative instruments
Number of derivative instruments | Instruments	0