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Fair Value of Financial Assets and Liabilities, Derivative Instruments (Details) MWh in Millions, MMBTU in Millions, $ in Millions | Dec. 31, 2018 USD ($) MMBTU MWh Counterparty | Dec. 31, 2017 MMBTU MWh
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Credit Concentration Risk |  |  |
Consideration of Credit Risk and Concentrations [Abstract] |  |  |
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty |  | 10 |
Credit Concentration Risk | Municipal or Cooperative Entities or Other Utilities [Member] |  |  |
Consideration of Credit Risk and Concentrations [Abstract] |  |  |
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty |  | 8 |
Credit Concentration Risk | External Credit Rating, Investment Grade [Member] |  |  |
Consideration of Credit Risk and Concentrations [Abstract] |  |  |
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty |  | 6 |
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties | $ |  | $ 96 |
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths) |  | 43.00% |
Credit Concentration Risk | No Investment Grade Ratings from External Credit Rating Agencies [Member] |  |  |
Consideration of Credit Risk and Concentrations [Abstract] |  |  |
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty |  | 3 |
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties | $ |  | $ 20 |
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths) |  | 9.00% |
Credit Concentration Risk | Internal Noninvestment Grade [Member] |  |  |
Consideration of Credit Risk and Concentrations [Abstract] |  |  |
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty |  | 1 |
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties | $ |  | $ 12 |
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths) |  | 5.00% |
Interest Rate Swap |  |  |
Interest Rate Derivatives [Abstract] |  |  |
Amount of accumulated other comprehensive gains (losses) related to interest rate derivatives expected to be reclassified into earnings within the next twelve months | $ |  | $ (3) |
Derivative Liability, Notional Amount | $ |  | $ 300 |
Electric Commodity (in megawatt hours) |  |  |
Gross Notional Amounts of Commodity Forwards, Options and FTRs [Abstract] |  |  |
Derivative, Nonmonetary Notional amount | MWh | [1],[2] | 87 | 68
Natural Gas Commodity (in million British thermal units) |  |  |
Gross Notional Amounts of Commodity Forwards, Options and FTRs [Abstract] |  |  |
Derivative, Nonmonetary Notional amount | MMBTU | [1],[2] | 92 | 37
[1] | Amounts are not reflective of net positions in the underlying commodities.
[2] | Notional amounts for options are included on a gross basis, but are weighted for the probability of exercise.

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