Fair Value of Financial Assets and Liabilities - Consideration of Credit Risk Concentrations (Details) - Credit Concentration Risk $ in Millions	Jun. 30, 2020 USD ($) Counterparty
Derivative [Line Items]
Significant counterparties	9
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 | $	$ 14.8
Credit exposure, percentage	36.00%
Internal investment grade
Derivative [Line Items]
Significant counterparties	6
Credit exposure | $	$ 26.0
Credit exposure, percentage	64.00%
External Credit Rating, Non Investment Grade [Member]
Derivative [Line Items]
Significant counterparties	1
Credit exposure | $	$ 0.1
Credit exposure, percentage	0.20%