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Discontinued Operations, Acquisitions and Dispositions - Rockford Disposition (Details) $ in Millions |  |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jul. 12, 2016 USD ($) | May 12, 2016 USD ($) MW | Jun. 30, 2016 USD ($) | Dec. 31, 2017 USD ($) MW | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Generation capacity (in MW) | MW |  |  |  | 30,000 |  |
Impairment losses |  |  |  | $ 1,709 | $ 702 | $ 4,860
Rockford |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Percentage of ownership interest |  | 100.00% |  |  |  |
Impairment losses |  |  |  |  | $ 17 |
Proceeds from sale |  | $ 55 |  |  |  |
Discontinued Operations, Held-for-sale or Disposed of by Sale | Rockford |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Percentage of ownership interest |  | 100.00% |  |  |  |
Cash consideration |  | $ 55 |  |  |  |
Generation capacity (in MW) | MW |  | 450 |  |  |  |
Impairment losses |  |  | $ 17 |  |  |
Proceeds from sale | $ 56 |  |  |  |  |
Base Residual Auction Results Adjustments | $ 1 |  |  |  |  |

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