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DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Derivative assets    
Other current derivative assets $ 35.3 $ 6.7
Other long-term derivative assets 4.8 0.2
Total derivative assets 40.1 6.9
Derivative liabilities    
Other current derivative liabilities 12.6 24.8
Other long-term derivative liabilities 2.9 4.1
Total derivative liabilities 15.5 28.9
Natural gas contracts    
Derivative assets    
Other current derivative assets 31.1 3.4
Other long-term derivative assets 4.5 0.0
Derivative liabilities    
Other current derivative liabilities 4.7 21.8
Other long-term derivative liabilities 0.1 0.9
FTRs    
Derivative assets    
Other current derivative assets 4.0 3.1
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 1.3 0.2
Other long-term derivative liabilities 1.1 0.0
Interest rate swaps    
Derivative assets    
Other current derivative assets 0.0 0.0
Other long-term derivative assets 0.0 0.0
Derivative liabilities    
Other current derivative liabilities 6.6 2.8
Other long-term derivative liabilities $ 1.7 $ 3.2