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 |