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Derivative Instruments and Hedging Activities - Pro Forma Impact Of Offsetting Assets (Details) - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Derivative Asset [Abstract]    
Gross asset $ 83.3 $ 134.8
Gross asset 83.3 134.8
Pro forma net presentation, asset, total 17.8 46.2
Counterparties with Offsetting Positions or Collateral [Member]    
Derivative Asset [Abstract]    
Gross asset 75.5 118.9
Pro forma net presentation, asset 10.0 30.3
Counterparties without Offsetting Position [Member]    
Derivative Asset [Abstract]    
Gross asset 7.8 15.9
Pro forma net presentation, asset, total 7.8 15.9
Current Assets from Risk Management Activities [Member]    
Derivative Asset [Abstract]    
Gross asset 76.8 85.5
Pro forma net presentation, asset, current 17.8 20.1
Current Assets from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member]    
Derivative Asset [Abstract]    
Gross asset 69.0 81.1
Pro forma net presentation, asset 10.0 15.7
Current Assets from Risk Management Activities [Member] | Counterparties without Offsetting Position [Member]    
Derivative Asset [Abstract]    
Gross asset 7.8 4.4
Pro forma net presentation, asset, current 7.8 4.4
Long-Term Assets from Risk Management Activities [Member]    
Derivative Asset [Abstract]    
Gross asset 6.5 49.3
Pro forma net presentation, asset, noncurrent   26.1
Long-Term Assets from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member]    
Derivative Asset [Abstract]    
Gross asset $ 6.5 37.8
Pro forma net presentation, asset   14.6
Long-Term Assets from Risk Management Activities [Member] | Counterparties without Offsetting Position [Member]    
Derivative Asset [Abstract]    
Gross asset   11.5
Pro forma net presentation, asset, noncurrent   $ 11.5