Fair Value of Financial Assets and Liabilities Consideration of Credit Risk Concentrations (Details) - Credit Concentration Risk $ in Millions	Mar. 31, 2020 USD ($) Counterparty
Derivative [Line Items]
Significant counterparties	10
Municipal or cooperative electric entities or other utilities
Derivative [Line Items]
Significant counterparties	9
External credit rating, investment grade
Derivative [Line Items]
Significant counterparties	2
Credit exposure | $	$ 11.5
Credit exposure, percentage	32.00%
Internal investment grade
Derivative [Line Items]
Significant counterparties	6
Credit exposure | $	$ 23.3
Credit exposure, percentage	65.00%
External Credit Rating, Non Investment Grade [Member]
Derivative [Line Items]
Significant counterparties	2
Credit exposure | $	$ 0.6
Credit exposure, percentage	2.00%