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DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) - USD ($)
$ in Millions
Sep. 30, 2019
Dec. 31, 2018
Derivative assets    
Other current derivative assets $ 15.4 $ 15.3
Other long-term derivative assets 0.3 0.6
Total derivative assets 15.7 15.9
Derivative liabilities    
Other current derivative liabilities 24.9 5.8
Other long-term derivative liabilities 6.7 2.1
Total derivative liabilities 31.6 7.9
Natural gas contracts    
Derivative assets    
Other current derivative assets 8.9 7.7
Other long-term derivative assets 0.0 0.4
Derivative liabilities    
Other current derivative liabilities 22.3 5.3
Other long-term derivative liabilities 2.2 0.2
FTRs    
Derivative assets    
Other current derivative assets 6.2 7.4
Derivative liabilities    
Other current derivative liabilities 0.0 0.0
Coal contracts    
Derivative assets    
Other current derivative assets 0.3 0.2
Other long-term derivative assets 0.3 0.2
Derivative liabilities    
Other current derivative liabilities 0.1 0.1
Other long-term derivative liabilities 0.1 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 2.5 0.4
Other long-term derivative liabilities $ 4.4 $ 1.9