Fair Value of Financial Assets and Liabilities - Consideration of Credit Risk Concentrations (Details) - Credit Concentration Risk $ in Millions	Sep. 30, 2021 USD ($) Counterparty
Derivative [Line Items]
Significant counterparties	8
Municipal or cooperative electric entities or other utilities
Derivative [Line Items]
Significant counterparties	8
External credit rating, investment grade
Derivative [Line Items]
Significant counterparties	2
Credit exposure | $	$ 11
Credit exposure, percentage	29.00%
Internal investment grade
Derivative [Line Items]
Significant counterparties	5
Credit exposure | $	$ 27
Credit exposure, percentage	70.00%
External Credit Rating, Non Investment Grade [Member]
Derivative [Line Items]
Significant counterparties	1