DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) $ in Millions	Sep. 30, 2020 USD ($) Instruments	Dec. 31, 2019 USD ($)
Derivative assets
Other current derivative assets	$ 6.4	$ 1.9
Other long-term derivative assets	1.0	0.2
Total derivative assets	7.4	2.1
Derivative liabilities
Other current derivative liabilities	0.7	3.1
Other long-term derivative liabilities	0.4	0.2
Total derivative liabilities	1.1	3.3
Natural gas contracts
Derivative assets
Other current derivative assets	4.3	0.4
Other long-term derivative assets	0.7	0.0
Derivative liabilities
Other current derivative liabilities	0.4	3.1
Other long-term derivative liabilities	0.0	0.1
FTRs
Derivative assets
Other current derivative assets	1.9	1.3
Derivative liabilities
Other current derivative liabilities	0.0	0.0
Coal contracts
Derivative assets
Other current derivative assets	0.2	0.2
Other long-term derivative assets	0.3	0.2
Derivative liabilities
Other current derivative liabilities	0.3	0.0
Other long-term derivative liabilities	$ 0.4	$ 0.1
Derivatives designated as hedging instruments
Derivative instruments
Number of derivative instruments | Instruments	0