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] .