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Derivative Instruments and Hedging Activities, Pro Forma Impact - Offsetting Collateral (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Derivative Asset [Abstract]    
Gross collateral $ 39.0 $ 22.9
Counterparties with Offsetting Positions or Collateral [Member]    
Derivative Asset [Abstract]    
Gross collateral 39.0 22.9
Current Assets from Risk Management Activities [Member]    
Derivative Asset [Abstract]    
Gross collateral 39.0 22.9
Current Assets from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member]    
Derivative Asset [Abstract]    
Gross collateral $ 39.0 $ 22.9