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Derivative Financial Instruments - Summary of AOCI related to Cash Flow Hedges (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Derivative [Line Items] |  |  |  |  |  |
Beginning Balance |  |  | $ (3,026) |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | [1] |  | (182) | $ (144) |  |
Ending Balance |  | $ (2,995) | (2,995) | (3,026) |  |
Income Tax Expense (Benefit) |  | (26) | (120) | 126 |  | $ (753)
Exelon Generation Co L L C [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Beginning Balance |  |  | (37) |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | [1] |  | (1) | (4) |  |
Ending Balance |  | (38) | (38) | (37) |  |
Income Tax Expense (Benefit) |  |  | 108 | 1,376 |  | (282)
Cash Flow Hedging [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Beginning Balance |  |  | (14) | (17) |  |
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net |  |  | 11 | (1) |  |
Ending Balance |  | (2) | (2) | (14) |  | (17)
Cash Flow Hedging [Member] | Interest Expense [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  |  | 1 | 4 | [2] |
Cash Flow Hedging [Member] | Exelon Generation Co L L C [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Beginning Balance |  |  | (16) | (19) |  |
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net |  |  | 11 | (1) |  |
Ending Balance |  | $ (4) | (4) | (16) |  | $ (19)
Cash Flow Hedging [Member] | Exelon Generation Co L L C [Member] | Interest Expense [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  |  | $ 1 | 4 | [2] |
Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash Flow Hedging [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Income Tax Expense (Benefit) |  |  |  | $ 1 |  |
[1] | Amounts in parenthesis represent a decrease in net income.
[2] | Amount is net of related income tax expense of $1 million for the year ended December 31, 2017.

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- Definition Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e637-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e681-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580 + Details
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- Definition The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4D -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624177-113959 + Details
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