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Mergers, Acquisitions and Dispositions Merger Transaction Table (Details) - USD ($) $ in Millions |  | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 23, 2016 | Dec. 31, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Payments to Acquire Businesses, Gross |  |  | $ 154 | $ 208 | $ 6,923
Pepco Holdings [Member] |  |  |  |  |
Payments to Acquire Businesses, Gross | $ 7,142 |  |  |  |
Pepco Holdings LLC [Member] | Pepco Holdings LLC [Member] | Deferred Compensation, Share-based Payments [Member] |  |  |  |  |
Stockholders' Equity, Other |  | $ 29 |  |  |

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