Fair Value of Financial Assets and Liabilities Consideration of Credit Risk and Concentrations (Details) - Credit Concentration Risk $ in Millions	Dec. 31, 2019 USD ($) Counterparty
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure	10
Municipal or Cooperative Entities or Other Utilities
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure	9
External Credit Rating, Investment Grade
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure	3
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties | $	$ 12.2
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties	35.00%
No Investment Grade Ratings from External Credit Rating Agencies
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure	6
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties | $	$ 22.1
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties	65.00%
External Credit Rating, Non Investment Grade
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure	1
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties | $	$ 0.1
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties	1.00%