Asset Retirement Obligations - Rollforward of Nuclear Decommissioning ARO (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Asset Retirement Obligation Roll Forward Analysis [Line Items]
Disposal Group, Including Discontinued Operation, Assets, Current		$ 910		$ 910		$ 0
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Accretion Expense	[1]			367	$ 350
Exelon Generation Co L L C [Member]
Asset Retirement Obligation Roll Forward Analysis [Line Items]
Disposal Group, Including Discontinued Operation, Assets, Current		910		910		0
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Accretion Expense	[1]			365	$ 350
Net increase due to changes in, and timing of, estimated future cash flows				345
Exelon Generation Co L L C [Member] | Nuclear Decommissioning [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Nuclear Decommissioning ARO, Beginning Balance	[2]		$ 9,662	9,662
Asset Retirement Obligation, Liabilities Settled				(783)
Accretion Expense				357
Net increase due to changes in, and timing of, estimated future cash flows		84	$ 32	116
Costs incurred related to decommissioning plants				(35)
Nuclear Decommissioning ARO, Ending Balance		9,317		9,317
Current portion of ARO		$ 12		$ 12		$ 13

[1] Included in Operating and maintenance expense on the Registrants’ Consolidated Statements of Operations and Comprehensive Income.

[2] Includes $12 million and $13 million for the current portion of the ARO at September 30, 2018 and December 31, 2017, respectively, which is included in Other current liabilities on Exelon’s and Generation’s Consolidated Balance Sheets.