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Acquisitions and Dispositions (Narrative) (Detail) $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2015 USD ($) Project | Jun. 30, 2015 USD ($) Project MW | Sep. 30, 2016 USD ($) MW | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Nov. 30, 2019 | Nov. 30, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |
Project Cost |  |  |  | $ 151 | $ 405 | $ 40 |  |
White River Hub [Member] |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |
Minority Interest Ownership Percentage By Parent |  |  |  | 50.00% |  |  |  |
Iroquois Gas Transmission System Lp [Member] |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |
Minority Interest Ownership Percentage By Parent |  |  |  | 25.93% |  |  |  |
Subsequent Event [Member] | Cove Point |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |
Noncontrolling partnership percentage interest |  |  |  |  |  |  | 25.00% |
Cove Point |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |
Ownership percentage |  |  |  | 75.00% |  |  |  |
Four Brothers Solar, LLC |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |
Percentage of equity interests acquired |  | 50.00% |  |  |  |  |  |
Number of Projects | Project |  | 4 |  |  |  |  |  |
Purchase price |  | $ 64 |  |  |  |  |  |
MW capacity | MW |  | 320 |  |  |  |  |  |
Project Cost |  |  | $ 670 |  |  |  |  |
Four Brothers Solar, LLC | Investment Credits | IRS |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |
Percentage of federal investment tax credits on projects claimed |  |  |  | 99.00% |  |  |  |
Three Cedars |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |
Percentage of equity interests acquired | 50.00% |  |  |  |  |  |  |
Number of Projects | Project | 3 |  |  |  |  |  |  |
Purchase price | $ 43 |  |  |  |  |  |  |
MW capacity | MW |  |  | 210 |  |  |  |  |
Project Cost |  |  | $ 450 |  |  |  |  |
NRG Energy, Inc |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |
Contributions to Four Brothers and Three Cedars |  |  |  |  | 9 |  |  |
NRG Energy, Inc | Four Brothers Solar, LLC |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |
Percentage of equity interests acquired |  |  |  |  |  |  |  | 50.00%
Sun Edison Inc |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |
Contributions to Four Brothers and Three Cedars |  |  |  |  |  | $ 189 |  |
Sun Edison Inc | Four Brothers Solar, LLC |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |
Contributions to Four Brothers and Three Cedars |  |  |  |  | $ 301 |  |  |

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