Consideration of Credit Risk Concentrations (Details) - Credit Concentration Risk $ in Millions	Sep. 30, 2020 USD ($) Counterparty
Derivative [Line Items]
Significant counterparties	10
Municipal or cooperative electric entities or other utilities
Derivative [Line Items]
Significant counterparties	10
External credit rating, investment grade
Derivative [Line Items]
Significant counterparties	3
Credit exposure | $	$ 17.9
Credit exposure, percentage	41.00%
Internal investment grade
Derivative [Line Items]
Significant counterparties	6
Credit exposure | $	$ 25.5
Credit exposure, percentage	59.00%
External Credit Rating, Non Investment Grade [Member]
Derivative [Line Items]
Significant counterparties	1
Credit exposure | $	$ 0.1
Credit exposure, percentage	0.30%