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Fair Value Measurements (Details 2) - Level 3 $ in Millions | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2017 USD ($) MWh $ / MWh | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
FTRs |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value | $ | $ 3 | $ 1 | $ (5)
FTRs | FES |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value | $ | 0 | (1) | (6)
NUG contracts |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value | $ | (92) | $ (107) | $ (136)
Model | FTRs |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value | $ | 3 |  |
Model | FTRs | FES |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value | $ | 0 |  |
Model | NUG contracts |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value | $ | $ (92) |  |
Model | Minimum | FTRs |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) | (5.00) |  |
Model | Minimum | FTRs | FES |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) | (5.00) |  |
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) | 29.90 |  |
Model | Maximum | FTRs |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) | 4.60 |  |
Model | Maximum | FTRs | FES |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) | 2.60 |  |
Model | Maximum | NUG contracts |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, Power (in MWH) | MWh | 2,322,000 |  |
Fair Value Inputs, Power, Regional Prices (in $/MWH) | 31.40 |  |
Model | Weighted Average | FTRs |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) | 0.60 |  |
Model | Weighted Average | FTRs | FES |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) | 0.20 |  |
Model | Weighted Average | NUG contracts |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, Power (in MWH) | MWh | 470,000 |  |
Fair Value Inputs, Power, Regional Prices (in $/MWH) | 29.90 |  |

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