Fair Value of Financial Assets and Liabilities Consideration of Credit Risk Concentrations (Details) - Credit Concentration Risk $ in Millions	Sep. 30, 2019 USD ($) Counterparty
Derivative [Line Items]
Significant counterparties	6
Municipal or cooperative electric entities or other utilities
Derivative [Line Items]
Significant counterparties	6
External credit rating, investment grade
Derivative [Line Items]
Significant counterparties	1
Credit exposure | $	$ 14.2
Credit exposure, percentage	31.00%
Internal investment grade
Derivative [Line Items]
Significant counterparties	5
Credit exposure | $	$ 9.7
Credit exposure, percentage	21.00%