Derivative Instruments and Hedging Activities, Pro Forma Impact - Offsetting Collateral (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Derivative Asset [Abstract]
Gross collateral	$ 48.2	$ (4.9)
Counterparties with Offsetting Positions or Collateral [Member]
Derivative Asset [Abstract]
Gross collateral	48.2	(4.9)
Current Assets from Risk Management Activities [Member]
Derivative Asset [Abstract]
Gross collateral	48.2	(4.9)
Current Assets from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member]
Derivative Asset [Abstract]
Gross collateral	$ 48.2	$ (4.9)