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Derivative Financial Instruments - Summary of Proprietary Trading Activities (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Derivative [Line Items] |  |  |
Unrealized Gain (Loss) on Derivatives | $ (294) | $ (151) | $ (24)
Exelon Generation Co L L C [Member] |  |  |
Derivative [Line Items] |  |  |
Unrealized Gain (Loss) on Derivatives | (307) | (167) | (40)
Not Designated as Hedging Instrument, Trading [Member] | Exelon Generation Co L L C [Member] | Commodity Contract [Member] | Operating Revenue [Member] |  |  |
Derivative [Line Items] |  |  |
Unrealized Gain (Loss) on Derivatives | 17 | 6 | 2
Not Designated as Hedging Instrument, Trading [Member] | Exelon Generation Co L L C [Member] | Commodity Contract [Member] | Operating Revenue [Member] |  |  |
Derivative [Line Items] |  |  |
Unrealized Gain (Loss) on Derivatives | $ 0 | $ (1) | $ (1)

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- Definition The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period. + References Reference 1: 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 + Details
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