DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Derivative assets
Other current derivative assets	$ 78.2	$ 17.0
Other long-term derivative assets	8.6	0.9
Total derivative assets	86.8	17.9
Derivative liabilities
Other current derivative liabilities	9.3	20.5
Other long-term derivative liabilities	0.1	1.6
Total derivative liabilities	9.4	22.1
Natural gas contracts
Derivative assets
Other current derivative assets	66.7	13.0
Other long-term derivative assets	7.7	0.7
Derivative liabilities
Other current derivative liabilities	5.8	12.9
Other long-term derivative liabilities	0.0	1.2
FTRs
Derivative assets
Other current derivative assets	5.4	2.4
Derivative liabilities
Other current derivative liabilities	0.0	0.0
Coal contracts
Derivative assets
Other current derivative assets	6.1	1.6
Other long-term derivative assets	0.9	0.2
Derivative liabilities
Other current derivative liabilities	0.1	0.8
Other long-term derivative liabilities	0.1	0.4
Interest rate swaps
Derivative assets
Other current derivative assets	0.0	0.0
Derivative liabilities
Other current derivative liabilities	$ 3.4	$ 6.8