Asset Retirement Obligations - Nuclear Decommissioning Asset Retirement Obligation Rollforward (Details) - USD ($) $ in Millions		3 Months Ended					9 Months Ended	12 Months Ended
		Dec. 31, 2017		Sep. 30, 2016	Mar. 31, 2016		Sep. 30, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Accretion Expense	[1]							$ 468		$ 446		$ 398
Atlantic City Electric Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Accretion Expense	[1]							0		0		0
Exelon Generation Co L L C [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Accretion Expense	[1]							468		446		397
Asset Retirement Obligation, Revision of Estimate							$ 61	1		1
Asset Retirement Obligation, Period Increase (Decrease)		$ 180		$ 165			288			488
Nuclear decommissioning | Exelon Generation Co L L C [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
ARO beginning balance					$ 8,246		8,246	8,734	[2]	8,246
Asset Retirement Obligation, Accretion Expense								458		436
Asset Retirement Obligation, Revision of Estimate								444
Asset Retirement Obligation, Period Increase (Decrease)								34		61
Costs incurred to decommission retired plants								(8)		(9)
ARO ending balance		9,662						9,662		8,734	[2]	8,246
Asset Retirement Obligation, Current		13						13		10
Nonnuclear Decommissioning Asset Retirement Obligation [Member] | Atlantic City Electric Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
ARO beginning balance					0		0	3		0
Asset Retirement Obligation, Accretion Expense								0		0
Asset Retirement Obligation, Revision of Estimate					0	[3]		0	[4]	3	[4]
Asset Retirement Obligation, Period Increase (Decrease)								0		0	[5]
ARO ending balance		3						3		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					197		$ 197	199	[6]	197
Asset Retirement Obligation, Accretion Expense								10		10
Asset Retirement Obligation, Revision of Estimate					$ 1	[3]		(1)	[4]	8	[4]
Asset Retirement Obligation, Period Increase (Decrease)								7		(22)	[5]
ARO ending balance		197	[6]					197	[6]	199	[6]	$ 197
Asset Retirement Obligation, Current		$ 1						$ 1		$ 1

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

[2] Includes $13 million and $10 million as the current portion of the ARO at December 31, 2017 and 2016, respectively, which is included in Other current liabilities on Exelon’s and Generation’s Consolidated Balance Sheets.

[3] Following the completion of the PHI merger on March 23, 2016, PHI's AROs related to its unregulated business interests were transferred to Exelon and Generation.

[4] During the year ended December 31, 2017, ComEd recorded a decrease of $1 million in Operating and maintenance expense. Generation, PECO, BGE, Pepco, DPL and ACE did not record any adjustments in Operating and maintenance expense for the year ended December 31, 2017. During the year ended December 31, 2016, Generation recorded a increase of $1 million in Operating and maintenance expense. ComEd, PECO, BGE, Pepco, DPL and ACE did not record any adjustments in Operating and maintenance expense for the year ended December 31, 2016.

[5] Reflects a reduction to the ARO resulting primarily from the sales of the New Boston generating site and Upstream business in 2016. See Note 4—Mergers, Acquisitions and Dispositions for further information.

[6] Excludes the current portion of the ARO at December 31, 2017 for Generation, ComEd and BGE of $1 million, $2 million and $2 million, respectively. Excludes the current portion of the ARO at December 31, 2016 for Generation, ComEd and BGE of $1 million, $2 million and $3 million, respectively. This is included in Other current liabilities on the Registrants' respective Consolidated Balance Sheets.