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Fair Value Measurements (Details 2) - Level 3 $ in Millions | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2016 USD ($) MWh $ / MWh | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($)
FTRs |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value | $ | $ 6 | $ (5) | $ 25
FTRs | FES |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value | $ | 2 | (6) | 14
NUG contracts |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value | $ | (118) | $ (136) | $ (151)
Model | FTRs |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value | $ | 6 |  |
Model | FTRs | FES |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value | $ | 2 |  |
Model | NUG contracts |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value | $ | $ (118) |  |
Model | Minimum | FTRs |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) | (2.20) |  |
Model | Minimum | FTRs | FES |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) | (2.20) |  |
Model | Minimum | NUG contracts |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, Power (in MWH) | MWh | 400 |  |
Fair Value Inputs, Power, Regional Prices (in $/MWH) | 30.90 |  |
Model | Maximum | FTRs |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) | 7.60 |  |
Model | Maximum | FTRs | FES |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) | 7.60 |  |
Model | Maximum | NUG contracts |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, Power (in MWH) | MWh | 3,207,000 |  |
Fair Value Inputs, Power, Regional Prices (in $/MWH) | 35.30 |  |
Model | Weighted Average | FTRs |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) | 1.00 |  |
Model | Weighted Average | FTRs | FES |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) | 0.70 |  |
Model | Weighted Average | NUG contracts |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, Power (in MWH) | MWh | 661,000 |  |
Fair Value Inputs, Power, Regional Prices (in $/MWH) | 32.10 |  |

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