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Intangible Assets - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Nov. 01, 2016 | Mar. 24, 2016
Goodwill [Line Items] |  |  |  |  |
Goodwill | $ 6,677 | $ 6,677 | $ 6,677 |  |
Amortization of Intangible Assets | (109) | (237) | (336) |  |
Exelon Generation Co L L C [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Goodwill | 47 | 47 |  |  |
Amortization of Intangible Assets | 63 | 83 | 79 |  |
Commonwealth Edison Co [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Goodwill | 2,625 | 2,625 | 2,625 |  |
Amortization of Intangible Assets | 7 | 7 | 7 |  |
Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount |  |  |  | 30.00% |
Pepco Holdings LLC [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Goodwill | 4,005 | 4,005 | $ 4,005 |  |
Potomac Electric Power Company [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets |  |  |  |  | $ 2,100
Fresh-Start Adjustment, Increase (Decrease), Goodwill |  |  |  |  | 400
Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount |  |  |  | 30.00% |
Delmarva Power & Light Company [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Goodwill | $ 8 | $ 8 |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets |  |  |  |  | 1,400
Fresh-Start Adjustment, Increase (Decrease), Goodwill |  |  |  |  | 300
Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount |  |  |  | 20.00% |
Atlantic City Electric Company [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets |  |  |  |  | 500
Fresh-Start Adjustment, Increase (Decrease), Goodwill |  |  |  |  | $ (700)
Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount |  |  |  | 30.00% |
Constellation Energy Group LLC [Member] | Trade Names [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Useful life | 10 years |  |  |  |

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- Definition The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275 + Details
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