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Derivative Instruments - Derivative assets and liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Derivative Asset    
Other current derivative asset $ 11.8 $ 38.2
Other long-term derivative assets 0.6 3.4
Derivative asset 12.4 41.6
Derivative Liability    
Other current derivative liabilities 10.0 1.9
Other long-term derivative Iiabilities 1.6 0.5
Derivative liability 11.6 2.4
Natural gas contracts    
Derivative Asset    
Other current derivative asset 5.6 31.4
Other long-term derivative assets 0.1 2.9
Derivative Liability    
Other current derivative liabilities 9.4 0.4
Other long-term derivative Iiabilities 1.4 0.0
Petroleum products contracts    
Derivative Asset    
Other current derivative asset 1.2 0.2
Derivative Liability    
Other current derivative liabilities 0.0 0.1
FTRs    
Derivative Asset    
Other current derivative asset 4.4 5.1
Derivative Liability    
Other current derivative liabilities 0.0 0.0
Coal contracts    
Derivative Asset    
Other current derivative asset 0.6 1.5
Other long-term derivative assets 0.5 0.5
Derivative Liability    
Other current derivative liabilities 0.6 1.4
Other long-term derivative Iiabilities $ 0.2 $ 0.5