DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) $ in Millions	Jun. 30, 2021 USD ($) Instruments	Dec. 31, 2020 USD ($)
Derivative assets
Other current derivative assets	$ 30.7	$ 6.2
Other long-term derivative assets	2.5	0.1
Total derivative assets	33.2	6.3
Derivative liabilities
Other current derivative liabilities	0.4	3.7
Other long-term derivative liabilities	0.0	0.4
Total derivative liabilities	0.4	4.1
Natural gas contracts
Derivative assets
Other current derivative assets	22.4	3.7
Other long-term derivative assets	2.0	0.1
Derivative liabilities
Other current derivative liabilities	0.4	3.2
Other long-term derivative liabilities	0.0	0.3
FTRs
Derivative assets
Other current derivative assets	2.6	1.1
Derivative liabilities
Other current derivative liabilities	0.0	0.0
Coal contracts
Derivative assets
Other current derivative assets	5.7	1.4
Other long-term derivative assets	0.5	0.0
Derivative liabilities
Other current derivative liabilities	0.0	0.5
Other long-term derivative liabilities	$ 0.0	$ 0.1
Derivatives designated as hedging instruments
Derivative instruments
Number of derivative instruments | Instruments	0