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Derivative Instruments and Hedging Activities - Pro Forma Impact Of Offsetting Assets (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Derivative Asset [Abstract]    
Gross asset $ 75.9 $ 61.1
Derivative assets 75.9 61.1
Pro forma net presentation, asset, total 28.7 28.6
Counterparties with Offsetting Positions or Collateral [Member]    
Derivative Asset [Abstract]    
Gross asset 75.9 61.1
Pro forma net presentation, asset 28.7 28.6
Current Assets from Risk Management Activities [Member]    
Derivative Asset [Abstract]    
Gross asset 54.2 37.9
Pro forma net presentation, asset, current 19.7 13.8
Current Assets from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member]    
Derivative Asset [Abstract]    
Gross asset 54.2 37.9
Pro forma net presentation, asset 19.7 13.8
Long-Term Assets from Risk Management Activities [Member]    
Derivative Asset [Abstract]    
Gross asset 21.7 23.2
Pro forma net presentation, asset, noncurrent 9.0 14.8
Long-Term Assets from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member]    
Derivative Asset [Abstract]    
Gross asset 21.7 23.2
Pro forma net presentation, asset $ 9.0 $ 14.8