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DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) - USD ($)
$ in Millions
Mar. 31, 2020
Dec. 31, 2019
Derivative assets    
Other current derivative assets $ 4.1 $ 6.7
Other long-term derivative assets 1.7 0.2
Total derivative assets 5.8 6.9
Derivative liabilities    
Other current derivative liabilities 27.9 24.8
Other long-term derivative liabilities 4.9 4.1
Total derivative liabilities 32.8 28.9
Natural gas contracts    
Derivative assets    
Other current derivative assets 3.0 3.4
Other long-term derivative assets 1.6 0.0
Derivative liabilities    
Other current derivative liabilities 22.7 21.8
Other long-term derivative liabilities 0.0 0.9
FTRs    
Derivative assets    
Other current derivative assets 0.9 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 0.1 0.2
Derivative liabilities    
Other current derivative liabilities 0.1 0.2
Other long-term derivative liabilities 0.0 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 5.1 2.8
Other long-term derivative liabilities $ 4.9 $ 3.2