Fair Value of Financial Assets and Liabilities, Derivative Instruments (Details)
MWh in Thousands, $ in Millions |
Sep. 30, 2017
USD ($)
MWh
|
Dec. 31, 2016
MWh
|
| 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 |
|
8
|
|
| 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 |
|
5
|
|
| Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties |
|
$ 9.2
|
|
| Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths) |
|
20.00%
|
|
| 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 |
|
1
|
|
| Credit Concentration Risk | Credit Quality Less Than Investment Grade [Member] |
|
|
|
| Consideration of Credit Risk and Concentrations [Abstract] |
|
|
|
| Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties |
|
$ 0.2
|
|
| Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths) |
|
1.00%
|
|
| 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 |
|
2
|
|
| Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties |
|
$ 15.3
|
|
| Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths) |
|
33.00%
|
|
| Electric Commodity (in megawatt hours) |
|
|
|
| Gross Notional Amounts of Commodity FTRs [Abstract] |
|
|
|
| Derivative, Nonmonetary Notional amount | MWh |
[1] |
6,183
|
2,685
|
|
|