DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Derivative assets
Current derivative assets	$ 247.1	$ 17.0
Long-term derivative assets	23.0	0.9
Total derivative assets	270.1	17.9
Derivative liabilities
Current derivative liabilities	22.3	20.5
Long-term derivative liabilities	0.0	1.6
Total derivative liabilities	22.3	22.1
Natural gas contracts
Derivative assets
Current derivative assets	204.7	13.0
Long-term derivative assets	17.5	0.7
Derivative liabilities
Current derivative liabilities	20.6	12.9
Long-term derivative liabilities	0.0	1.2
FTRs
Derivative assets
Current derivative assets	3.7	2.4
Derivative liabilities
Current derivative liabilities	0.0	0.0
Coal contracts
Derivative assets
Current derivative assets	38.7	1.6
Long-term derivative assets	5.5	0.2
Derivative liabilities
Current derivative liabilities	0.0	0.8
Long-term derivative liabilities	0.0	0.4
Interest rate swaps
Derivative assets
Current derivative assets	0.0	0.0
Derivative liabilities
Current derivative liabilities	$ 1.7	$ 6.8