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Derivative Instruments and Hedging Activities - Pro Forma Impact Of Offsetting Assets (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Derivative Asset [Abstract]    
Gross asset $ 99.0 $ 149.4
Derivative assets 99.0 149.4
Pro forma net presentation, asset, total 68.0 116.4
Counterparties with Offsetting Positions or Collateral [Member]    
Derivative Asset [Abstract]    
Gross asset 88.0 108.9
Pro forma net presentation, asset 57.0 75.9
Counterparties without Offsetting Position [Member]    
Derivative Asset [Abstract]    
Gross asset 11.0 40.5
Pro forma net presentation, asset, total 11.0 40.5
Current Assets from Risk Management Activities [Member]    
Derivative Asset [Abstract]    
Gross asset 75.9 115.3
Pro forma net presentation, asset, current 50.4 85.3
Current Assets from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member]    
Derivative Asset [Abstract]    
Gross asset 69.0 100.0
Pro forma net presentation, asset 43.5 70.0
Current Assets from Risk Management Activities [Member] | Counterparties without Offsetting Position [Member]    
Derivative Asset [Abstract]    
Gross asset 6.9 15.3
Pro forma net presentation, asset, current 6.9 15.3
Long-Term Assets from Risk Management Activities [Member]    
Derivative Asset [Abstract]    
Gross asset 23.1 34.1
Pro forma net presentation, asset, noncurrent 17.6 31.1
Long-Term Assets from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member]    
Derivative Asset [Abstract]    
Gross asset 19.0 8.9
Pro forma net presentation, asset 13.5 5.9
Long-Term Assets from Risk Management Activities [Member] | Counterparties without Offsetting Position [Member]    
Derivative Asset [Abstract]    
Gross asset 4.1 25.2
Pro forma net presentation, asset, noncurrent $ 4.1 $ 25.2