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Derivative Instruments and Hedging Activities - Pro Forma Impact Of Offsetting Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Derivative Asset [Abstract]    
Gross asset $ 134.8 $ 138.8
Gross asset 134.8 138.8
Pro forma net presentation, asset, total 46.2 79.8
Counterparties with Offsetting Positions or Collateral [Member]    
Derivative Asset [Abstract]    
Gross asset 118.9 133.1
Pro forma net presentation, asset 30.3 74.1
Counterparties without Offsetting Position [Member]    
Derivative Asset [Abstract]    
Gross asset 15.9 5.7
Pro forma net presentation, asset, total 15.9 5.7
Current Assets from Risk Management Activities [Member]    
Derivative Asset [Abstract]    
Gross asset 85.5 103.3
Pro forma net presentation, asset, current 20.1 59.5
Current Assets from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member]    
Derivative Asset [Abstract]    
Gross asset 81.1 99.8
Pro forma net presentation, asset 15.7 56.0
Current Assets from Risk Management Activities [Member] | Counterparties without Offsetting Position [Member]    
Derivative Asset [Abstract]    
Gross asset 4.4 3.5
Pro forma net presentation, asset, current 4.4 3.5
Long-Term Assets from Risk Management Activities [Member]    
Derivative Asset [Abstract]    
Gross asset 49.3 35.5
Pro forma net presentation, asset, noncurrent 26.1 20.3
Long-Term Assets from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member]    
Derivative Asset [Abstract]    
Gross asset 37.8 33.3
Pro forma net presentation, asset 14.6 18.1
Long-Term Assets from Risk Management Activities [Member] | Counterparties without Offsetting Position [Member]    
Derivative Asset [Abstract]    
Gross asset 11.5 2.2
Pro forma net presentation, asset, noncurrent $ 11.5 $ 2.2