Derivative Instruments and Hedging Activities, Pro Forma Impact - Offsetting Collateral (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Derivative Asset [Abstract]
Gross collateral	$ (7.3)	$ (14.2)
Counterparties with Offsetting Positions or Collateral [Member]
Derivative Asset [Abstract]
Gross collateral	(7.3)	(14.2)
Current Assets from Risk Management Activities [Member]
Derivative Asset [Abstract]
Gross collateral	(7.3)	(14.2)
Current Assets from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member]
Derivative Asset [Abstract]
Gross collateral	$ (7.3)	$ (14.2)