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Stock-Based Compensation Plans - Narrative (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Common Stock Narrative Information [Line Items] |  |  |  |
Obligations of outstanding restricted stock not settled | $ 83 |  | $ 108 |
Settlement of restricted stock | 106 |  | $ 88 | $ 68
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options | $ 33 |  |  |
Weighted Average Period Non Vested Performance Share Awards Are Expected To Be Recognized Over | 1 year 8 months |  |  |
Performance Shares [Member] |  |  |  |
Common Stock Narrative Information [Line Items] |  |  |  |
Granted, Weighted Average Grant Date Fair Value (in usd per share) | $ 38.15 | [1] | $ 35.00 | $ 28.85
Percentage to be settled as common stock | 50.00% |  |  |
Percentage to be settled as cash | 50.00% |  |  |
Percentage to be settled as cash if ownership requirements are met | 100.00% |  |  |
Performance Shares [Member] | Pepco Holdings LLC [Member] |  |  |  |
Common Stock Narrative Information [Line Items] |  |  |  |
Granted, Weighted Average Grant Date Fair Value (in usd per share) |  |  |  | 26.10
Employee Stock Option [Member] |  |  |  |
Common Stock Narrative Information [Line Items] |  |  |  |
Requisite service period | 4 years |  |  |
Restricted Stock [Member] |  |  |  |
Common Stock Narrative Information [Line Items] |  |  |  |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition | 1 year 8 months 12 days |  |  |
Restricted Stock Units (RSUs) [Member] |  |  |  |
Common Stock Narrative Information [Line Items] |  |  |  |
Granted, Weighted Average Grant Date Fair Value (in usd per share) | $ 38.60 |  | $ 34.98 | $ 28.14
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options | $ 38 |  |  |
Minimum [Member] | Restricted Stock [Member] |  |  |  |
Common Stock Narrative Information [Line Items] |  |  |  |
Requisite service period | 3 years |  |  |
Maximum [Member] | Restricted Stock [Member] |  |  |  |
Common Stock Narrative Information [Line Items] |  |  |  |
Requisite service period | 5 years |  |  |
LTIP [Member] |  |  |  |
Common Stock Narrative Information [Line Items] |  |  |  |
Shares authorized | 11 |  |  |
[1] | (a)As of December 31, 2018, $33 million of total unrecognized compensation costs related to nonvested performance shares are expected to be recognized over the remaining weighted-average period of 1.7 years.

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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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 (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation. + 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 (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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 (a)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + 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 (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + 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 (a)(3) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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