Fair Value of Financial Assets and Liabilities Consideration of Credit Risk and Concentrations (Details) - Credit Concentration Risk $ in Millions	Dec. 31, 2019 USD ($) Counterparty
Derivative [Line Items]
Number of most significant counterparties	10
Municipal or Cooperative Entities or Other Utilities
Derivative [Line Items]
Number of most significant counterparties	9
External Credit Rating, Investment Grade
Derivative [Line Items]
Number of most significant counterparties	6
Credit exposure for the most significant counterparties | $	$ 154
Percentage of credit exposure for the most significant counterparties	60.00%
Internal Investment Grade
Derivative [Line Items]
Number of most significant counterparties	4
Credit exposure for the most significant counterparties | $	$ 37
Percentage of credit exposure for the most significant counterparties	14.00%