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Stock-Based Compensation Plans - Components of Tax Benefits from Stock-based Compensation (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 23, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items] |  |  |  |
Tax benefit on stock compensation |  |  |  | $ (18)
Restricted Stock Units (RSUs) [Member] |  |  |  |
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items] |  |  |  |
Tax benefit on stock compensation |  | $ (28) | $ (35) | (27)
Performance Shares [Member] |  |  |  |
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items] |  |  |  |
Tax benefit on stock compensation |  | $ (16) | $ (29) | $ (18)
Pepco Holdings LLC [Member] | Predecessor [Member] |  |  |  |
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items] |  |  |  |
Tax benefit on stock compensation | $ (1) |  |  |

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- Definition The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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