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Asset Retirement Obligations - Nuclear Decommissioning Asset Retirement Obligation Rollforward (Details) - USD ($) | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2018 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |  |  |  |  |  |  |  |
Asset Retirement Obligation, Accretion Expense | [1] |  |  |  | $ 489,000,000 |  | $ 468,000,000 |  | $ 446,000,000
Asset Retirement Obligation, Revision of Estimate |  |  |  |  | 20,000,000 |  |  |  | 2,000,000
Atlantic City Electric Company [Member] |  |  |  |  |  |  |  |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |  |  |  |  |  |  |  |
Asset Retirement Obligation, Accretion Expense | [1] |  |  |  | 0 |  | 0 |  | 0
Asset Retirement Obligation, Revision of Estimate |  |  |  |  | (1,000,000) |  |  |  | 2,000,000
Exelon Generation Co L L C [Member] |  |  |  |  |  |  |  |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |  |  |  |  |  |  |  |
Asset Retirement Obligation, Accretion Expense | [1] |  |  |  | 489,000,000 |  | 468,000,000 |  | 446,000,000
Asset Retirement Obligation, Revision of Estimate |  | $ 203,000,000 | $ 84,000,000 | $ 138,000,000 | 0 |  |  |  | 0
Asset Retirement Obligation, Period Increase (Decrease) |  |  |  | 180,000,000 |  |  |  |  | 34,000,000
Nuclear decommissioning |  |  |  |  |  |  |  |  |
Asset Retirement Obligation Roll Forward Analysis [Line Items] |  |  |  |  |  |  |  |  |
Proceeds from Customers |  |  |  |  | 144,000,000 |  | 77,000,000 |  |
Nuclear decommissioning | Exelon Generation Co L L C [Member] |  |  |  |  |  |  |  |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |  |  |  |  |  |  |  |
ARO beginning balance |  |  |  | 8,734,000,000 | 9,662,000,000 | [2] | 8,734,000,000 |  |
Asset Retirement Obligation, Accretion Expense |  |  |  |  | 478,000,000 |  | 458,000,000 |  |
Asset Retirement Obligation, Revision of Estimate |  |  | $ 772,000,000 |  |  |  | 444,000,000 |  |
Asset Retirement Obligation, Period Increase (Decrease) |  |  |  |  | (77,000,000) |  | 34,000,000 |  |
ARO ending balance |  | 10,005,000,000 |  |  | 10,005,000,000 |  | 9,662,000,000 | [2] | 8,734,000,000
Asset Retirement Obligation, Current |  | 22,000,000 |  |  | 22,000,000 |  | 13,000,000 |  |
Asset Retirement Obligation, Liabilities Incurred |  |  |  |  | (58,000,000) |  | 8,000,000 |  |
Nonnuclear Decommissioning Asset Retirement Obligation [Member] |  |  |  |  |  |  |  |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |  |  |  |  |  |  |  |
Asset Retirement Obligation, Period Increase (Decrease) |  |  |  |  | (3,000,000) |  |  |  |
Nonnuclear Decommissioning Asset Retirement Obligation [Member] | Atlantic City Electric Company [Member] |  |  |  |  |  |  |  |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |  |  |  |  |  |  |  |
ARO beginning balance |  |  |  | 3,000,000 | 3,000,000 |  | 3,000,000 |  |
Asset Retirement Obligation, Accretion Expense |  |  |  |  | 0 |  | 0 |  |
Asset Retirement Obligation, Revision of Estimate |  |  |  |  | 1,000,000 |  | 0 |  |
Asset Retirement Obligation, Period Increase (Decrease) |  |  |  |  | 0 |  | 0 |  |
ARO ending balance |  | 4,000,000 |  |  | 4,000,000 |  | 3,000,000 |  | 3,000,000
Asset Retirement Obligation, Liabilities Incurred | [3] |  |  |  |  |  | 0 |  |
Nonnuclear Decommissioning Asset Retirement Obligation [Member] | Exelon Generation Co L L C [Member] |  |  |  |  |  |  |  |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |  |  |  |  |  |  |  |
ARO beginning balance |  |  |  | $ 199,000,000 | 197,000,000 |  | 199,000,000 |  |
Asset Retirement Obligation, Accretion Expense |  |  |  |  | 10,000,000 |  | 10,000,000 |  |
Asset Retirement Obligation, Revision of Estimate |  |  |  |  | 35,000,000 |  | (1,000,000) |  |
Asset Retirement Obligation, Period Increase (Decrease) |  |  |  |  | (3,000,000) |  | (7,000,000) |  |
ARO ending balance |  | $ 238,000,000 |  |  | $ 238,000,000 |  | 197,000,000 |  | $ 199,000,000
Asset Retirement Obligation, Liabilities Incurred | [3] |  |  |  |  |  | $ 1,000,000 |  |
[1] | Included in Operating and maintenance expense in the Registrants’ Consolidated Statements of Operations and Comprehensive Income.
[2] | Includes $22 million and $13 million as the current portion of the ARO at December 31, 2018 and 2017, respectively, which is included in Other current liabilities in Exelon’s and Generation’s Consolidated Balance Sheets.
[3] | .

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- Definition The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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