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DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Derivative Asset    
Other current derivative assets $ 21.6 $ 11.8
Other long-term derivative assets 1.4 0.6
Derivative asset 23.0 12.4
Derivative Liability    
Other current derivative liabilities 4.5 10.0
Other long-term derivative liabilities 0.4 1.6
Derivative liability 4.9 11.6
Natural gas contracts    
Derivative Asset    
Other current derivative assets 9.7 5.6
Other long-term derivative assets 0.2 0.1
Derivative Liability    
Other current derivative liabilities 4.5 9.4
Other long-term derivative liabilities 0.4 1.4
Petroleum products contracts    
Derivative Asset    
Other current derivative assets 0.0 1.2
Derivative Liability    
Other current derivative liabilities 0.0 0.0
FTRs    
Derivative Asset    
Other current derivative assets 11.5 4.4
Derivative Liability    
Other current derivative liabilities 0.0 0.0
Coal contracts    
Derivative Asset    
Other current derivative assets 0.4 0.6
Other long-term derivative assets 0.4 0.5
Derivative Liability    
Other current derivative liabilities 0.0 0.6
Other long-term derivative liabilities 0.0 0.2
Interest rate swaps    
Derivative Asset    
Other long-term derivative assets 0.8 0.0
Derivative Liability    
Other long-term derivative liabilities $ 0.0 $ 0.0